Attacking/Defending status

If two opposing armies are moving toward each other, when they meet they will both be attacking and defending.

What happens if both are moving in the same direction and one overtakes the other. Would the who gets hit from behind be in defending status only until attack is initiated?

Also when infantry defending against attacker and offensive friendly units come in to reenforce does that put all units in the attack and defend status or is there a separation? If subs attacked ships and destroyers came in to help, the ships would remain on defensive status, right?

And when disembarking with ships on same dot when enemy air shows up, being the ships and transports show as two groups, do ( if 10 of each ) both groups fire a 10 shot back at the planes while the planes damage is distributed over all?

( I still think a interceptor/strategic bomber combo hits transports for 3 times what it should. Needs checking )

While at it just to confirm, if planes are patrolling an enemy area and a few big armies are within the patrol, is it the top 10 of everything that shoots back or the top 10 of each group? If defender had interceptors that definitely would be an additional 10 shots on defense, right?

Man I think I knew some of this but the memory is going.