This work on sad people?

I would say yes but only strong sadness. They would still be a little bit sad.

Can you choose specific people to cast on or does it have to be everyone in range?

I believe that it is everyone in range.

Do both parts of the spell fall under the category of Charmed as stated in the Damage/Effect? Seeing as you are using a spell to affect someone’s Mind/Emotional State? Or is it a completely different effect called Indifference. Like how does it work when casting it on Elves or the like who have advantage against that.

Those tags are mostly for sorting I would assume.     No where in the spell does it say that the effectiveness of this spell is changed by someones immunity or resistance to charm or creature type. So I would assume that the elves Fey Ancestry would not apply here as that ability seems to be referring to the charmed condition. This spell does not charm anyone simple tweak their emotions in a direction the caster wants them, and it even can suppress charms.

But its always up to the DM in the end.

Do the people/person i want the creature to be not hostile to have to be within that 20 ft radius? Or can i choose any creature at any range that it's hostile towards?

Dr Sigmund Fraud, charlatan warlock of the Archfey, would like to cast this as therapy on the raging barbarian...

I plan on asking my DM if my artificer bard could infuse items with this spell and create the "first" well known substitute for weed just so I can make somewhat of a profit 

Would everyone say this works on beasts and mobs of low intelligence as well?  

It seems like RAW it's specifically humanoids, but if your DM says it's fine then why not?

Ask a Ceric or Barber Practitioner if CE is right for you.

Could Calm Emotions be used to drop a Barbarian out of rage?  I know it doesn't say so specifically but it seems like one could make strong arguments that this is the spell to do that.

RAW, no, at least not directly. The 2 options are suppress toan effect causing the charmed/frightened condition, or to make someone indifferent about creatures they are hostile to. Rage is not any of these so can't be suppressed, however if you cause the raging barbarian to have no hostile creatures around them, and they take no damage, they will drop out of their rage at the end of their turn.

Would a Barbarian's rage be cancelled while in the area, or is that the DM's ruling?

Quote from NeverGiveTheDMIdeas >>

Would a Barbarian's rage be cancelled while in the area, or is that the DM's ruling?

 There are no rules for or against it. DM's ruling. At my table, yes the rage would end

If it was used to make the barbarian indifferent to its enemies, and thus not attack them on its next turn, the barbarian would fall out of the rage.  So not directly, but potentially passively.

If my party member is charmed for example by a vampire and I cast this to suppress the charm. Would he be willing or trying to resist my spell?

Charmed only makes the target non-hostile to the caster.  It's not like mind-control.  In your scenario with your party member being charmed they're presumably not hostile towards you so I would rule it as being willing unless you give them cause to be hostile towards you or to believe you're about to hurt them.

Vampire charm adds additional influence beyond the base charm effect, though. The target "regards the vampire as a trusted friend to be heeded and protected", per the ability's description. Would probably take some kind of direct instruction from the vampire, but I could see it being reasonable. Ultimately it probably depends how hard the DM wants to push the party.