Scenario A: While adventuring in the Dark Dungeon, Lucy Lightbearer is wearing a light spell that provides 20' radius bright light plus 20' dim light. She's fighting Harry Human and his Hungry Henchmen. Alfie Archer is standing 50' away in the dark. Alfie has not attempted to hide.

True or false:
1. Lucy, Harry, and all the henchmen know where Alfie is, and can attack him with their crossbows whenever they like. Those attacks have disadvantage because Alfie is in darkness.
2. Alfie can attack Harry and the Henchmen. Those attacks have advantage because they can't see him in the dark.
3. If one of the henchmen has darkvision 60' they can see Alfie, but their attacks against him still have disadvantage because of the dim light where Alfie is standing.

Scenario B: While adventuring in the Dark Dungeon, Lucy Lightbearer is wearing a light spell that provides 20' radius bright light plus 20' dim light. She's fighting Harry Human and his Hungry Henchmen. Alfie Archer is standing 50' away in the dark. Alfie used an action to hide last round, and rolled a 19 stealth check.

True or false:
1. Lucy, Harry, and the henchmen don't know where Alfie is, and may not attack him without first making a perception check against DC 19. If they succeed, they can attack him with their crossbows whenever they like until Alfie makes another attempt to hide. Those attacks have disadvantage because Alfie is in darkness.
2. Lucy (passive perception 12) and the henchmen (passive perception 14) don't know where Alfie is, and may not attack him. Harry (passive perception 20) does know where Harry is, and can attack him with a crossbow at disadvantage.
3. Alfie can attack Harry and the Henchmen. Those attacks have advantage because they can't see him in the dark.
4. If Alfie attacks, Harry or any of the Henchmen, he reveals his location, and we're back to Scenario A.

Scenario A:

1) True, see rules for unseen attackers, unless his location has changed without them knowing (via sound or Archies attacking)

2) True, see rules for unseen attackers.  Archie also gives up his location if he attacks, even if they can’t see him

3) False, Dim light only lightly obscures Archie, and light obscurity does not grant disadvantage to attacks.  Archies attacks against the dark vision using henchman would also not have advantage

scenario B:

Archie hiding is duplicating the effect of darkness, so really there wouldn’t be a change unless he did something to physically alter his situation (like hide behind physical cover). So if he’s just “hiding” in the darkness and not behind an object...

1) False,  Archie has not effectively changed his state. He was unseen before, and he is unseen now, and the same rules apply for unseen attackers regardless of whether they are in darkness, invisible, or hidden.  Now if he was able to move without alerting his enemies, their attacks would automatically miss, but hiding is not movement

2) False, see answer to number 1

3) True, because similar scenario A situation is true

4) Irrelevant as there is no real change from scenario A

A lot going on here.  Technically, Alfie is in non magical darkness and, thus, heavily obscured which blocks all vision of him.  RAW a DM shouldn't ask for a check on seeing him, but you can try using hearing.  If they do, again RAW it should an action to Search for him without imposing disadvantage due to the darkness(other factors might apply), negating a possible attack this round.  IF you go a route that allows it and you do spot him, then

Scenario A

1. True
2. True
3. False - Darkvision would mean he sees to 60ft as though in dim light, which only imposes Disadvantage on Wisdom (Perception) checks.  There is no penalty to attack.

Scenario B

1. True
2. True
3. True
4. True, but I would rule that it is only the individuals who spotted him.

A lot of the confusion I experienced with these types of scenarios is that it follows the blinded condition, but how it is applied goes against common wisdom because of how it is applied.  A creature in bright light looking into darkness is Blinded and thus gets those effects applied to this scenario.  Odd that a creature in light is blinded, right?   You have a good grasp on the situation and I think your rulings were pretty fair.

A) 

1. Neither true nor false. They can attempt an attack at disadvantage no matter what, but they do not know for a fact whether Alfie is in any particular square.
2. True
3. False. Light Obscurement does not cause disadvantage to attack rolls. It causes disadvantage to Perception rolls. The henchmen with Darkvision can see Alfie, and make attacks as normal.

B)

1. Neither true nor false. Is Alfie also behind cover?
   • If not, Alfie is not hidden to the henchmen with Darkvision, and they may attack as normal. If Alfie is also in cover, then the hencmen with Darkvision must make a Perception check at disadvantage, but they are able to make such a check.
   • Lucy & Harry automatically fail Perception checks that rely on sight, but they are able to make such a check against any noise Alfie may be making. That's typically going to be a higher DC than normal.
2. True for Harry. For the others, same as above.
3. True only against those that don't have Darkvision. If Alfie is also successfully hidden, whether anyone has Darkvision or not, Alfie still gets advantage from that.
4. True

Scenario A:

1. True
2. True
3. False, light obscurement does not give disadvantage.

Scenario B:

1. Partially false (assuming passive perception < 19). They may attack a square they think he's in. If they're right, they attack with disadvantage as normal; if they're wrong they automatically miss.
2. See the above. Also, Harry can tell them what square to attack, or they can use Wait to attack wherever Harry attacks.
3. True, unless Harry has Darkvision as well as Passive Perception 20.
4. True

Hm.  Curious to see all these answers that basically equate to "darkness doesn't mean spit."

As I would rule it to my best understanding of the spirit and letter of the rules:

Scenario A:
1.) False(ish).  If Alfie has done nothing to give away his position yet, his opponents do not know where he is.  In effect, he benefits from the Hide action until he does something that necessitates otherwise.  I would rule this as a case of passive Stealth vs. passive Perception (at disadvantage due to darkness and thus being sound-only perception).  The Hide action allows Alfie to invoke an active stealth roll and to attempt to break contact with anyone who's made him.  If Alfie Archer attacks, his opponents will know he's there, but they must attack a grid square, not "Alfie", and hope Alfie is in it.  Or in the case of me running this, they would indicate the line of travel for their shot, I would have them roll at disadvantage, and if they shot in the right direction and rolled well enough they would hit Alfie and hear that, indicating to them his position.

2.) True.  Alfie can see his targets clearly, and they cannot see him to defend themselves.  Attacks at advantage.

3.) True.  Darkvision treats dim light as normal light and darkness as dim light.  If Alfie is standing in darkness, as per the Dark Dungeon, then an attack against a target in dim light invokes disadvantage.  This henchman still has a huge edge in that he can see Alfie, and thus target him directly rather than firing into the darkness and guessing, even if his attack is at disadvantage.

Scenario B:
1.) True, though I don't particularly care for it.  Perception would likely be rolled at disadvantage due to the darkness involved.
2.) Again, true though distasteful.  If Harry Human is human, ears aren't enough to target crossbow fire.  Nevertheless, the rules are unfortunately clear.  Cases like this are why 5e should just bite the bullet and dispense with darkness/darkvision rules altogether.  Their normal rules almost completely defang darkness as a threat, which is just not how it should be.
3.) True, same reasoning as before.  
4.) True.

Quote from Blueluck >>

Scenario A: While adventuring in the Dark Dungeon, Lucy Lightbearer is wearing a light spell that provides 20' radius bright light plus 20' dim light. She's fighting Harry Human and his Hungry Henchmen. Alfie Archer is standing 50' away in the dark. Alfie has not attempted to hide.

True or false:
1. Lucy, Harry, and all the henchmen know where Alfie is, and can attack him with their crossbows whenever they like. Those attacks have disadvantage because Alfie is in darkness.
2. Alfie can attack Harry and the Henchmen. Those attacks have advantage because they can't see him in the dark.
3. If one of the henchmen has darkvision 60' they can see Alfie, but their attacks against him still have disadvantage because of the dim light where Alfie is standing.

Scenario B: While adventuring in the Dark Dungeon, Lucy Lightbearer is wearing a light spell that provides 20' radius bright light plus 20' dim light. She's fighting Harry Human and his Hungry Henchmen. Alfie Archer is standing 50' away in the dark. Alfie used an action to hide last round, and rolled a 19 stealth check.

True or false:
1. Lucy, Harry, and the henchmen don't know where Alfie is, and may not attack him without first making a perception check against DC 19. If they succeed, they can attack him with their crossbows whenever they like until Alfie makes another attempt to hide. Those attacks have disadvantage because Alfie is in darkness.
2. Lucy (passive perception 12) and the henchmen (passive perception 14) don't know where Alfie is, and may not attack him. Harry (passive perception 20) does know where Harry is, and can attack him with a crossbow at disadvantage.
3. Alfie can attack Harry and the Henchmen. Those attacks have advantage because they can't see him in the dark.
4. If Alfie attacks, Harry or any of the Henchmen, he reveals his location, and we're back to Scenario A.

Answers should be grounded in a citation to Chapter 8 or another relevant source, or its just all of us clamoring with different unfounded opinions.

• A1: DEBATABLE. We know that creatures standing in darkness cause other creatures to behave as though Blinded in relation to them. A blinded creature "automatically fails any ability check that requires sight." "Noticing danger, finding hidden objects, hitting an enemy in combat, and targeting a spell, to name just a few--rely heavily on a character's ability to see" from that same section. Contrasted with Blindsight, which allows a creature to "perceive its surroundings without relying on sight," we can conclude that normal creatures do rely on sight to perceive their surroundings? So:
  • If knowing what square an enemy is in is "noticing danger," and if that noticing relies on sight, and if that noticing can be considered an ability check like every other task in D&D seems to be (even ones of trivial difficulty), then a blinded creature "automatically fails" said check, meaning that a combatant would not automatically know what square that Alfie is in.
  • The weakest link in that chains are (1) is perceiving your surroundings (including noticing danger and where people are in combat) really an ability check?, and (2) does "rely heavily on" = "require sight"? If either of those are not true, then there's no reason to conclude that being Blinded would automatically cause you to fail to know where someone is in general.
  • Also, Chapter 9 provides that " If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side," suggesting that creatures automatically notice each other (i.e. know where each other are), unless there are active stealth rolls being made. This section would probably be superceded by any interaction with Blinded above, however.
  • Conclusion: if your dm considers regular vision to be a constant passive perception check with a trivial difficulty, then darkness obscures location and FALSE. If not, or if knowing a combatants position is partially based on non-vision senses (hearing), then everyone knows Alfie's position and TRUE.
• A2: TRUE. Darkness creates heavy obstruction. Heavy obstruction imposes Blinded when perceiving something "in that area," not when looking out of it. Alfie is in darkness, his targets are not, so they are Blinded in relation to him but he is not in relation to them. Attacking a Blinded target grants advantage, and a Blinded creature attacks others with disadvantage. 
• A3: FALSE. Darkvision treats Darkness as Dim Light within a certain radius. Dim Light is treated as Light Obstruction. Light Obstruction imposes "disadvantage on Wisdom (Perception) checks that rely on sight," not on attacks. (Also note, Light Obstruction is worded differently than Heavy Obstruction, so it's not entirely clear whether this disadvantage is on checks to perceive something within the obstruction, or also applies to a creature within the Light Obstruction looking out. Probably RAI for it to just apply in and not out, same as Heavy Obstruction?).
• B1: TRUE. And to be clear, succeeding those Perception checks would not merely put them back to A1 above, it would instead create a state where they clearly have used a non-vision sense to perceive him, and so there's no longer a possibility that they can't find him as a result of being Blinded. 
• B2: TRUE, see B1 for Harry. 
• B3: TRUE, see A2 above. And also, Alfie is also receiving advantage from being hidden-Unseen (see Chapter 9), in addition to having advantage for attacking a Blinded combatant. He may have already had that, since all the various vision rules are hopelessly circular and self-referential, but now its doubly clear. 
• B4: DEBATABLE. Once he attacks, he is no longer Hidden: "If you are hidden--both unseen and unheard--when you make an attack, you give away your location when the attack hits or misses." And in that respect he's back to Scenario A. But if his location is unknown for a reason other than the fact that he was hiding (see A1 uncertainty), then losing hidden shouldn't also impact that other overlapping cause of being in an unknown square? 

Chicken Champ, on your A1 response, many of those things you mention (particularly attacks) are not ability checks.  the unseen attackers rule clearly implies you can attack a creature you can’t see, you just have disadvantage while doing so

Combatants often try to escape their foes' notice by hiding, casting the invisibility spell, or lurking in darkness.

When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see. If the target isn't in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target's location correctly.

When a creature can't see you, you have advantage on attack rolls against it.

If you are hidden — both unseen and unheard — when you make an attack, you give away your location when the attack hits or misses.

it also says you give away your position when you attack.  So knowing the position is possible even if you can’t see a creature.  Being blinded still allows you to attack as well, so to say that you can’t attack a creature in darkness isn’t accurate.  That said, if the creature in darkness moved in a way that was not detectable and didn’t attack, the enemy attacking it would not know it’s location and would have to guess.  But if Archie attacked and didn’t move, or moved in an audible manner, any enemy would know his location and could target him (with disadvantage) without having to guess, per the RAW for unseen attackers.  

Heh.  Thanks, CC.  That post is a wonderful demonstration of how nonsensical the vision/darkness rules are for 5e.  Clear as mud - just the way the DMG intended.

The vast number of different answers demonstrates why trying to write game rules in common language instead of technical terminology is risky.

Quote from iconarising >>

Chicken Champ, on your A1 response, many of those things you mention (particularly attacks) are not ability checks.  the unseen attackers rule clearly implies you can attack a creature you can’t see, you just have disadvantage while doing so

Combatants often try to escape their foes' notice by hiding, casting the invisibility spell, or lurking in darkness.

When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see. If the target isn't in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target's location correctly.

When a creature can't see you, you have advantage on attack rolls against it.

If you are hidden — both unseen and unheard — when you make an attack, you give away your location when the attack hits or misses.

it also says you give away your position when you attack.  So knowing the position is possible even if you can’t see a creature.  Being blinded still allows you to attack as well, so to say that you can’t attack a creature in darkness isn’t accurate.  That said, if the creature in darkness moved in a way that was not detectable and didn’t attack, the enemy attacking it would not know it’s location and would have to guess.  But if Archie attacked and didn’t move, or moved in an audible manner, any enemy would know his location and could target him (with disadvantage) without having to guess, per the RAW for unseen attackers.  

 Again, hard to draw bright lines, because as written its hard to tell the difference between being hidden, being behind a heavy obstruction, being invisible, or your target being Blind, they all do kind of the same thing. But don't paraphrase and accidentally overstate it: if you are hidden, then you give away your location when the attack hits or misses. If you're not hidden then the attack doesn't give away your location. So if there is indeed another reason that a location is unknown separate and apart from whether or not that creature is hidden, that cause is not overridden by an attack. Hidden makes clear that it needs both (1) unseen and (2) unheard, so Hidden might get broken merely because you are heard? But if the general Blindness/Obstruction/vision rules provide that you by default don't know where an unseen target is, then losing Hidden wouldn't change that. 

But I"m not saying 100% it's that way. Just, it's debatable, and unclear because of the fuzzy imprecise language that's used everywhere. 

Equally risky is writing game rules in language and terms nobody can understand, especially when you're specifically building an edition to try and appeal to the masses and the lowest common denominator.

Seems more like 5e didn't want darkness to be a big deal or a problem, but because of the detritus of older editions they kept the vestiges of various light level rules in the game for those who wanted it to matter.  For some people it's a big deal; light has come into play in a big way a couple of times in my own games and my players know better than to take it for granted.  For others it's handwaved, nobody ever checks or cares unless someone does something like cast Darkness.

Also, the fact that Darkness creates "magical darkness" which behaves entirely different from regular "darkness" (you can't see "through" magical darkness, meaning its a specific exception to the regular Heavy Obstruction rules which allow you to see through/out of it; Darkvision doesn't work in it). There are so many examples where the rules are made more complex by their insistence on reusing common words, rather than just creating new terms where needed (like maybe call magical darkness "inkiness" or "gloom" or something, so that it's more clear which spells are creating it and what class features interact with it?). 

Quote from Yurei1453 >>

Equally risky is writing game rules in language and terms nobody can understand, especially when you're specifically building an edition to try and appeal to the masses and the lowest common denominator.

 It's perfectly possible to have rules that are both clear and understandable. It just involves a particular writing style and may involve creating terminology for edge cases (for example, rather than using 'see', you want a term that means "Detect with a sense capable of being used to accurately attack").

I would rather say, "locate and attack normally".  I have to write technical specifications all the time and it becomes very cumbersome when dealing with folks that are trying to twist the words to mean something else.  I think D&D is best when everyone is in the same "spirit."