Welcome willkzhang, i’ll try to answer your questions:
1. i guess it depends on who is playing, some people play better in a team, some better alone, and some are more skilled or smart than others. Try to balance the number of players with their skills, so that both sides are more or less equal to eachother.
Personally if it is 1 against 2, i would let the 1 player be the Allies.
2. yes
3. 1 carrier can carry up to 2 planes (fighters and/or tactical bombers)
1 transporter can carry 1 inf + 1 other ground unit (inf/art/tank/mech/AA)
4. 2 (and the 2nd region can be a friendly one)
5. ehh…
6. the disadvantage for the attacker is that the landed ground units cannot retreat. Defenders still get to fire back (but not at the bombarding ships)
Hope this is helpful.