Unfortunately you can't edit any of the mission, all you can do is set parameters in the "training" section.
As for firing when in freefall....
You can fire your guns whilst damaged a lot (holes in wings, oil on windscreen) but once you'e been shot down then thats it,your effectively out of the fight until you respawn.
Don't worry about parachutes either
they do no harm or damage to your plane. Nor can you shoot them down.
You'll fInd games on xbox 360, its just about finding them at the right time. I don't know the unlock list from memory, but alot of unlocks are fairly easy to pick up.
In training mode the extra bombs/rockets are pretty useless anyway and serve no purpose.And you can't use any of you unlocks in the single player campaign or in single missions