1. As long as the group of the tanks is in safe mode, they'll drive in a column along the road. To do this, type this setbehaviour "safe" in the init field of the leader of the tank group.
2. There are a lot of ways to make air support come in after a radio signal. One way is to spawn the choppers in real time, or you could just have a couple choppers sitting around somewhere, with their current waypoint requiring a radio signal to be completed. If you have a move waypoint, once they're finished idleing (When the waypoint is completed), they'll take off and head to their next waypoint.
The simplest way to do this is to put your chopper group in a secret place somewhere on the ground. Name the choppers (e.g. chopper1, chopper2), and in each chopper's init field put this setfuel 0. Give them a "Move" waypoint to the target area, then perhaps make a loop around the area so that the gunships will circle it. Now make a trigger, in the field that chooses who activates the trigger, select "Radio Alpha" and in the On Activation field put chopper1 setfuel 1;chopper2 setfuel 1. Now in the player's init field put 1 setradiomsg "Call Choppers". Now you have a radio option (0-0-1) that reads "Call Choppers". If you choose it, the choppers will take off and head along their waypoint path.
Note - You can do this much more efficiently with a script that creates the choppers and sends them to a location specified with a map-click.
P.S. - If you want to, you can PM me with any questions you've got about scripting.