I can answer all your questions but will answer the ones I know the answer to.

Can stealth fighters enter a nation undetected without provoking war? Yes

What exactly can AWACS see? And the UAV? Any unit with the binoculars in the unit desription (including AWACS and UAV) can see stealth units in range. including rangers, stealth planes (bombers, fighters) some units gain that after certain research point some units have that right away

Are ASW choppers good to destroy surface ships? Yes they do more considerably more damage (like twice as much) to subs but they do damage surface ships as well

Is there any way to protect my infrastructure (not my population) from missiles? AA can destroy missiles Theatre defense I find best but any thing can hit the missile even fighters. But it has to do enough damage to take it to 0. So stacking is essential so if you have 1 theatre defense it porbably wont take it down but if you have 5 it will destroy the missile and if someone has multiple missiles you have to have multiple stacks AA can typically only attack once every hour. So if they fire 2 missiles you need 2 stacks.


Is there any way to end the game via agreement/ceasefire/inactiviy of certain amount of players? when there are only 3 active players you have the option to end game all 3 (or 2) must agree in the paper you have that option

Giving my opninion on this too^^

UAV: They can reveal the exact units, similar to helicopters. While most aircrafts can only see whether there is support, tank or infantry, the UAV can reveal the exact unit and numbers. Very useful.

AWACs: Awacs can't see anything more than "regular" aircrafts (e.g. Air Superiority Fighter). They have a huge radar range in which they uncover all radar contacts (except underwater ones). At max level the radar range is bigger than Spain

ASW Chopppers: I have never used them against surface ships. But according to the stats, they do quite a bit of damage. But the ships also do quite a bit of damage
ASW Choppers are certainly useful to sink unprotected transport ships or corvettes.
At max level they deal 6,5dmg to ships. Destroyers(4.1) & Cruisers(5) deal less damage, but they have a lot more HP. 5 damage to an ASW chopper is 25% HP gone. 6,5dmg to cruisers isn't even noticable due to Cruisers 75HP.

You can protect your infrastructure from missiles with AA units. If its primarly your homeland cities buildings, you usually can use TDS very well. Their huge AA range can protect multiple cities from missiles.
If you want to protect cruicial airfields further at the front I recommend "Mobile Anti Air". Their defense against missile is pretty good and they are much more mobile than TDS.
Always remember: To shoot down a missile, you need to have enough fire power to drain its HP to 0^^.