Why do you insist on making air units weak
Because aircraft in the civ engine have the potential to be massively unbalanced, particularly when only a handful of civs have good access to them. Think how massively powerful bombers are in vanilla civ when the other player doesn't have access to any aircraft. They are a game-changing unit that can win you the game by themselves, basically making you immune to any invasion and making it so you hardly lose any units on offense; and, unlike magic or siege units, they have range and flexibility.
Also, keep in mind that the strength of a bomber unit matters only when it is intercepted; it doesn't really directly effect how much damage it does.
So, aircraft units need to be weak enough that they take significant damage or die when intercepted by archer or aircraft units.
Scereamers are in the same unit slot as the other demons (bloodletters, daemonettes, plaguebearers and screamers). The screamer thus needs to have similar power to the other 3 units - and no other single unit can be as game-changing as a bomber.
Isen't the entire point of chaos to have crazily strong late units (greater demons, demon princes,... and such) and to balance it, a weak economy and urban structure, weak city health and the like.
No, chaos factions will have a similar economy to other factions (though without any of the nice booster traits like financial, agricultural, industrial, imperialistic, organized, etc.), and their late-game nits shouldn't be particularly more powerful than those of other factions. However, if they just go for the chaos magics line, then they miss out on economy things from the trade and infrastructure lines
Greater demons should be on an order of magnitude to dragons, and lesser demons to the other powerful tier3 units (knights, handgunners, etc.).
The most likely candiate for a demon prince model is the Hyborem model from FFH. If that thing isn't a Bloodthirster I don't know what is, but definitely post anything in particular that you have.