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.
