RAI/RAW - Gaseous Form Spell

I have some questions about how Gaseous Form works.

Scenario: The Level 5 party is up against some giant red dragon - "Smaug the Lesser".  Everyone rolls initiative.  The Owlin ranger fires an arrow, it misses.  The Bugbear paladin charges forward, but gets knocked prone and takes a massive hit point loss.  The Dragonborn cleric fires sacred flame and uses a healing word on the paladin.  Its not the best start.  Then the Tortle Wizard cracks his knuckles.  He runs right up to the dragon's head near the paladin, casts gaseous form, and then enters the dragon's nostrils and goes down into his lungs.

DM: Wait, you can't do that.
Wizard: Why not?
DM: . . . . . UMMM . . . .
Wizard: I wait until my next turn, then turn back into my natural form, and then magic missile his lungs to pudding.  Night night, lizzy.
DM: ...... UMMM .....
Wizard: The dragon passes out.  Can't breathe.  I shout out "Hey amigos, can you cut me out of this caiman bruja?"
DM ..... UMMMM......

- - - - - - - - -

QUESTIONS (LOL):

1. Can Gaseous Form allow you to enter inside the mouth/nostrils of a larger creature and then occupy their lungs?
2. While in gaseous form, If you occupy another creature's space but remain on the outside, can the creature attack you without leaving the space?
3. While in gaseous form and in another creature's space, can a third creature hit you but not the creature?  Or would they attack both simultaneously?  For instance, the Wizard envelops the Paladin.  The dragon strikes the Paladin, but also hits the Wizard?  Or let's say the Wizard in gaseous form occupies the dragon's space - could the ranger fire arrows through the gaseous form and hit the dragon?
4. If you normally have a movement of 30 feet, move 20 feet, and then enter into gaseous form, would you still be able to hover an additional 10 feet with the new flying speed, or would it be reduced to 1/3 (3-4 feet) since you already expended your movement?
5. Would a monster know that the "cloud" is its prey, or what it think it simply vanished in a puff of smoke?  How should you handle this RAW/RAI?
6. If the Tortle Wizard held his breath first (they can do this for one hour) would they still be able to hold the same breath after converting back, or would they have to breathe in again?  This seems pretty important if you go down into the belly instead of the lungs.
7. Gaseous Form says that it treats liquids like solids.  So I assume that if you went down into a monster and it was full of water or whatever, you'd be stuck?
8. Are there any other limitations to Gaseous Form that are not explicitly stated in the spell description, but are de facto impacted by other spells/mechanics I am not thinking of?

Thank you all in advance.

Firstly, Gaseous Form is a really, really BAD spell.  So you are going to be disapointed.  Sorry.

1) They can enter enter the same space as another creature.  No attacks, so the gaseous creature cannot even grapple.  I see no direct problem with claiming you are inside their body, but I would also state they can take a 5 ft step and this would leave you behind.  Note, the opponent cannot carry you in their arms and they cannot carry you in their lungs.

2) Yes, you can attack someone that is inside your space.  Typically this happens with different sized creatures - a Tiny creature can easily be in the same square a Large creature with little penalty to either one.

3) That would be a DM ruling.  I would treat it the same as a man riding a horse and allow someone to target whichever creature they wanted to (the gaseous or the solid).  I might give a partial cover penalty.

4) RAW Movement rules are... silly.  You look at the total you have moved that round and if it exceeds your current speed, you can move.  Otherwise you cannot.  So it matters which order you do things. In the example you gave, you moved 20, then became gaseous and had 10 ft move, so you have 10-20 = none left. 

5) Arcana check.  Which if they are not proficient would be d20 plus their Int modifier.   I would set the DC as 10 + spell level = 12.

6) Belly, lungs, etc. are irrelevant.  The second the solid creature moved, you are left behind. I would also say that Misty Clouds do not need to breath.

7) You are not stuck unless the target creature attempts to grapple you (description they are holding their breath).   If they do, you can resist the grapple.

8a) General rule of spells: Spells that do not list damage do not do damage more than a spell of the same level, usually a save or nothing (or worse).

 8b) Gaseous forms are affected by wind spells.  I would rule that a Gaseous Form gets no save against the Gust of Wind spell as they count as vapor that is dispersed automatically.

Also entering the lungs of a RED dragon as a gas has to be a terrible idea. Even if the DM somehow allows it, the suffocation and holding breath rules still apply and the dragon could just murk your allies in the meanwhile.

Don’t forget that exhaling and not breathing in the sentient gas cloud is an option as well.

The lungs question is fundamentally outside the scope of the rules, and DM's call. It's essentially isomorphic to the sort of tricks people sometimes try with shapeshifting, and the basic answer is similar: no, because that's not the sort of game we're playing.

Once the DM says no to your attempt to break the game physics, demanding they justify it on the spot is out of line.

The description for the Fireball spell contains the following:

"Flammable objects in the area that aren’t being worn or carried start burning."

https://www.dndbeyond.com/spells/2618887-fireball

Why would they say that? Why does a backpack catch fire if its on teh ground, but is immune to catching fire if I'm wearing it? It makes no real-world sense.

Its because the definition of burning says: something burning "takes 1d4 Fire damage at the start of each of its turns. As an action, you can extinguish fire on yourself by giving yourself the Prone condition and rolling on the ground."

Which means, the enemy casts fireball, they do some great AOE damage, and then suddenly all your gear and clothes and armor are on fire, and you and all your stuff is taking damage, and you've got to spend your next action just putting the fire out. It becomes a LOT more powerful that way, too powerful.

I'd say that GaseousForm wouldn't allow the enemy to enter your lungs for the same reason the clothes and gear you're wearing dont start burning from a fireball: Because it would be way too powerful.  They're expecting to do it without the dragon making any kind of saving throw? And once inside the dragon, again, no saving throw for the dragon to get them out? It's basically puts the dragon in an inescapable situation. ANytime that happens, it's almost always way too powerful.

not to mention, wikipedia guestimates the lungs of a Brontosaurus are about 900 liters. If that were a perfect cube, it would be one meter (1 yard) on a side. But that would mean its all full of little airways and cavities, and the space would be too cramped for a medium sized creature to fit inside. The "Die Hard" view of the world would suggest dragons have huge ventilation ducts going through them so someone can crawl around in there from one organ to another. But the reality is more like a prairie dog colony. Big, huge internal volume. Absolutely impossible for a human to crawl through.