So I am curious how atb dictates in mission like this where its little time like what we are in now, 0 time as in we are shutdown, or we have enough time for a full volley in AA fire like with the reinforcement.
As a rule, AtB doesn't do any of this. The Mechs would literally just
appear on the board on Turn 1, in the designated drop zone. Or they would just appear in the designated zone when they arrive as reinforcements, and you wouldn't be able to do anything about it. I'm adding in the airdropping stuff in a manner that seems to be the most fair because a) it's fun, and b) we have the rules for it, so why not use them, and c) it's a bit of a brake on the Clan reinforcements being able to literally phase into existence *behind* your units if they win the initiative, which is what AtB would normally do. Very literally; you move your Mech somewhere near the board center, and then a reinforcement Hunchback IIC deploys onto the board directly behind you at Range 2, and doesn't count as having moved in any way, so guess how good it's double-tap attacks are? THAT'S what AtB normally does.
I'm not allowing the airdropping stuff to be shot at during pregame because a) we have to draw a line to start somewhere, b) getting stuff killed before game starts will right piss off OPFOR players, and c) we take long enough to get Mech moving and shooting as it is.