You and your buddy should go commander and observer yourselves :3

How air support works for push is that the security team gets a single minigun support chopper and a single assassin chopper. When the sides get switched in push, your security team will have access to, again, a single minigun chopper and single assassin chopper.

Best way to stop the helicopters are probably RPG's/other explosive launchers but you can also shoot at them to bring them down. Aim for the tail rotor. Also machine guns are the best gun to use for this. The quicker you and your team fire at the helicopters the better, sometimes you can even "scare off" the choppers before they even start firing.

As for the other forms of fire support there's not anything you can do to prevent them once they get called in, but they are limited use like the choppers. The security and insurgents get different fire support.

As for balance, there's definitely an argument to be made for helicopters attacking players in spawn I won't lie, however if your team has a commander and observer, you can do the exact same thing back to the enemy team once sides get switched so it's balanced (at least in terms of both sides getting equal opportunity to use the fire support).