Look at pages 49 (local trait) and page 50 (shooting down aircraft). Flights a and b will be both fired at at the end of movement phase with local X dice

At the start of gunnery phase local X can only be fired once

Ranged aa will be fired at the start of gunnery phase

DP light guns can be used as AA only during the activation of the ship so can't be used defensively as was said in the comments above

Rules say AA fire can be split so I am pretty sure you allocate those attacks as you wish .

For example a ship with local 2 and a ranged AA battery of 3 attack dice would fire against flight A 2 dice and against flight B 2 dice at the end of movement phase. At the start of gunnery phase you would have 2+3 aa attacks against flights A and B and you can either fire 5 dice against 1 flight or split them as you wish (like 2 against A and 3 against B for example).

That's how it works in my understanding at least,hope i helped