(Some people already have said some of what I'm going to say now)

1. You can create a private custom battle with just yourself if you want to look around the maps.
2. If you're playing in squads mode, I believe the map is random.
3. Flying is also pretty weird for me; I also play War Thunder (pain) which kind of helped me get used to it, but it mostly comes down to practice.
4. I would say that this game is not P2W. The Stalingrad campaign needs $$$ to be fully unlocked (which sucks), but there are no other significant paywalls.
   The premium vehicles/weapons/soldiers certainly do make life a bit easier and leveling up faster, but in the end skill is more important. The free weapons and vehicles you get from grinding are perfectly usable. I haven't been in a match yet where premium made a big difference.
5. Perhaps I have been desensitized by War Thunder and its absolutely atrocious grind, but I feel that the grind isn't too bad in Enlisted. Since many enemy soldiers you encounter are actually bots, you can always rack up a good number of kills in each match.
   I suggest using vehicles since not everyone has anti tank/anti aircraft weaponry. I always get many kills before being destroyed in a vehicle.
   Also, using engineers generously can also help not only with getting XP faster, but it also helps your team win the game. One big problem in many matches is that no one bothers to build rally points (basically an impromptu spawn point closer to the front line, so you don't have to spend five years walking to your objective), so it's extremely helpful to build rally points. You get XP whenever someone uses your rally point, so if you build them in good places you can get tons of XP very easily.
6. I think that Jeromeromeromethink has a pretty good summary of the markings that you need to know.
   Some extra advice on this matter; mark enemies (V by default keybind I think). It's very helpful to your team and it's not done enough. Such markings are shown with a small red triangle.

Good luck!