First off, your first problem was probably that with your three Urals, the first one was the leader. Now when he dies, then #2 takes over, and that command to stop is lifted when he takes command If the leader is in the second ural, and #2 is in the lead ural, then a trigger saying when #2 dies to stop, should work( I did this in one of my first missions).
Second, your new request of a convoy randomly shooseing what direction to go at an intersection...
AI is stupid. All computers and technology is stupid. A dog is much smarter. If your computer could, and you told it to go fetch the paper, it would walk outside, pick it up, and stay there holding the paper, waiting for a command to come back inside iwth the paper. If you told your dog to do the same thing, he would go outside, get the paper, and walk back inside to you, with the paper.
Anyways, you need to first decide a way tha the convoy is going to know which way to go. Makeign the AI randomly choose is more difficult than makeign a trigger to say... detect where the helicopter is, so the convoy goes the route leading away from the helicopter.
However you conclude to do this, it will be best to do it with a script, being that alot of triggers, waypoints, and so forth on the map, may cause lag.