Thanks for your interest!
+ The intent was that activated defenses can be attacked while hidden ones could not. I may have overlooked this in the rules.
+ The star is the target zone's score.
+ All units can only engage one target at a time in a phase, so the defense unit can engage only one of the two aircraft.
+ No, the area can be attacked without elliminating the AA present there.
+ Yes, once exhausted (v3) it can still be engaged by defensive fire. Aircraft have to survive the gauntlet.
+ Yes, defense aircraft can pursue and engage an enemy aircraft multiple times. Additionally they can share a border edge with an enemy fighter, but cannot attack it.