Question 1 is pretty straightforward. Check out the chapter on Harm (page 162), and it gives some relative stats for what levels of harm various weapons have.
Now, Molotovs are not really designed for hurting people, so much as setting fire to things. I would also argue that armor would help, at least in the short term. The big thing with chemical fires (and that includes burning gas) is that so long as it doesn't hit exposed skin, you aren't going to be burned that badly.
I'd treat it like a half strength grenade (2 harm area messy) and maybe a custom move for it to begin burning whatever's around and flammable. A countdown would be great for keeping track of the fire's spread.
Or just do a follow-up move of taking away his stuff (in this case his armor) as it's now merrily burning and no longer effective, or maybe a move of the fire spreading (announcing future badness).
2. To address the realism, I should point out that Molotovs are bad news for vehicles and buildings, but in open areas its usually something that maims rather than kills. Scars are part of the apocalypse world, and the Angel's right there to patch up. They're not exactly accurate weapons, the amount of gas in the average cocktail is going to hurt, but probably not kill, because a lot of that burning fuel is going all over the place.
And if you stay away from AP, your chopper and his gang probably got at least 1 armor to mitigate the effect.