Had a situation come up in a game today which felt a bit weird.

A Falcon fm with 2 Firestorms had one blast marker. A formation of thunderbolts attack it, and because the Firestorms are farthest from the thunderbolts, only one gets to shoot.

However, in the end phase, the thunderbolts, having been positioned a bit careless, zoom over the formation, changing what tanks are actually farthest from the enemy. Can the Firestorm that was suppressed before the original attack fire now? (As it is now the closest unit to the t-bolts.)

Yes.

It's really hard to suppress AA if there are multiple sources in one formation for both the approach and disengagement move.

And you check suppression at any point during the aircraft's movement, and not just where it ends up (at the point of attack or board edge for disengagement moves).

For ground flak yes...assess suppression along the entire approach and disengagement moves and not at the start/end point.

This is why you have to place the AC on your board edge and move as per the rules to assess what ground flak will be able to shoot.