Let’s start off with some tips:
Food is the only resource you can directly produce, it’s renewable, thus being able to manufacture a lot of it to trade is very valuable. With that in mind build food production in your starting base as soon as you can. In your second base build several food buildings so you have more food to trade for materials and tech. In my most recent game i stuffed 6 food buildings into my second base.
Purchase or steal a second aircraft as soon as you can. Synedrion has a fast 5 seater, New Jericho has a 7 seater, and Anu have an incredibly slow 8 seater. Ideally you’ll have built your own 6 seat aircraft for your teams but stealing one early on to handle recruiting runs, trade runs or just starting scans leaves your main craft and it’s crew free to do the important stuff.
Training centers in a base stack, i set up my first base as my training center. Use an empty aircraft to hire new recruits and haul them back to the training center to sit idle. Doing this i had a second team at level 6-7 before my first team hit level 6.
Ground vehicles aren’t needed but can at least be handy for base defense. I read if you have no crew at a base under attack you get no notification, park a ground vehicle there though and you have time to fly in a defense force before the base is destroyed. Though they are not needed it does beat having an undefended base sitting in the mist. 2 Vehicles can be parked at each base, and since they don’t use up your soldiers housing slots if you can afford to build one for an at risk base it can be worth it.
How many troops do you need? In my current game i have 3 fully crewed 6 seat aircrafts and feel the need for more, i just picked up my 6th base and feel really vulnerable as i cannot cover half the globe if the aliens get busy attacking. Grab any recruit you can afford any chance you get, park them in your training center and let them level up.
Citadels have a massive Scylla as the objective. It’s much larger and more dangerous than the artillery pandorens. Should be easy to spot as they are several stories tall and can walk around.
I’m guessing by “artistry” you mean “artillery” the long range attacks they have? If so that’s a Chiron, believe they are limited to 5 volleys before they run out of ammo. Not much you can really do if it’s out of range or line of sight of your snipers. Maybe put your guys under a roof and far enough in the bombs cannot catch them in the blast radius.
There are doors, they open when a unit walks adjacent to or through them and close at the end of your turn or the aliens turn, depending on which side opened the door. Doors can be destroyed by weapons though. If you want to set up an overwatch trap inside of a door pull your guys at least one square away from the doorframe before setting up overwatch so the door can close.
As Yokes said, a level 1 squad won’t survive on the field int he late game. Though it’s easy to level up new recruits with how training facilities stack if in the same base. Just be sure and move any new recruits to the base.
As for allying all factions i believe the answer is yes, though it’d take some tricky juggling of their missions to pull it off. Seems anytime you help one faction the others get annoyed with you. They also have relations with eachother, and over time it generally goes down until it becomes war. In my current game once Synedrion angered New Jericho enough NJ did a blitzkrieg on them. Something like 8 havens fell in a five minute period before i stepped in, though of course that hurt my relations with New Jericho… Humanity seems pretty intent on eradicating eachother rather than the pandorans here.