First of all, defensive AA of an aircraft. When can an aircraft that has carried out e.g. a ground attack use it’s defensive AA weapons? Any time an enemy air formation makes an approach move within their weapon range, or only against enemy interceptors targeting the formation?

And how do you play the disengagement move? Do the players take turns or does all aircrafts disengage simultaneously? If you take turns can air units use their AA attacks against disengaging enemies?

What about landed air-transports that opts to remain on the ground, I assume they can use their AA weapons against disengaging enemies? Do they fire with -1 to-hit for ”moving” this turn?

Finally, spaceships with either bombs or pin-point attacks. When does the player have to declare which weapon they will use? As part of the army list, before deployment or when the spaceship is activated? And if taken to a tournament can you change weaponry based on opponent?

Q: The rules state "Note that aircraft carrying out a ground attack mission that are armed with AA weapons may shoot at enemy interceptors that fall within the AA weapon’s fire arc." This sounds as if only aircraft on ground attack missions may use their flak attacks. Also, it seems to imply that the ground attack formation can only fire at interceptors targeting that formation. Can aircraft on ground attack missions fire at interceptors other than those attacking it? For example, if Bomber Flight 1 ends in the AA arc of Bomber Flight 2 and is intercepted by enemy fighters, can Bomber Flight 2 fire, even though it is not the target of the interceptors? Can aircraft on missions other than ground attack make flak attacks at all?
A: Yes to all the questions. If aircraft end their approach move within range and fire arc of AA weapons, the enemy get flak attacks regardless of the missions of any of the aircraft.

The definitive statement of how flak attacks work is in the first sentence, "[flak attacks may] shoot immediately after an enemy aircraft formation makes an approach move but before it makes its attack." The sentence quoted in the question is explanatory, intended to point out a particular situation that might not be immediately obvious (that's why it begins with "Note..."). It is not intended to createadditional restrictions on the basic flak attack mechanic.

As a side note, this has been a common point of contention. This is the NetEA answer and it is the ruling used by the Epic UK tournament organizers as well. Some groups house rule aircraft versus aircraft flak attacks differently.

I recall Ginger saying that an approach move doesn't count as a move for the purposes of triggering overwatch (although the disembarking of transported units does).