Recall, I said:
"in the last seconds of the map"
Meaning that, no matter when you trigger it, you will still have until the end of the map to clear the zone. It's like the escape zone in most maps, just because someone orders the helicopter does not mean the game is over. You stll have until the end of the map to get to the helicopter. Same with the airstrike. You can order the airstrike whenever the mission becomes active, but the airstrike won't start until the timer has expired.

And you could even make it so that it is not just a finale mission. If you make it on an announce system, the airstrike mission can be at anytime in the map. How cool would this be:
"Roger, This is command control, we acknoweldge your request for an airstrike on the designated target area. ETA on the airstrike is T minus 60 seconds. Please evacuate the target area now."
This way no matter what, even if one player ordered it without anyone knowing, you still will be notified via radio and every player will know and have 1 minute (or more or less) to clear the target area.