2)You can earn XP and money if you do PvE custom battle. Pick your favorite mission
3)The squad max size is 4, but a wing can be up to 12 i believe.
4)Yes and no. You will still be able to que for t1 battles. It will probably put you into a mixed battle, including both T1 and T2. There is a chance that you will be able to battle with your friends, however if you were in a squad with them it would put them into a mixed T1 and T2 battle if you are T1 or T2, but otherwise it would put them into the highest tier you have in your combat slots.