Not till they get further from civilians.
It's also wise to give the tail end of the convoy time to get far enough away from the city that the number of minutes it takes them to retreat exceeds the number of minutes it takes us to smoke the entire convoy.
Not a factor. You hit the rear first, then the front. The rest are stuck in the middle. As on the GW-1 Highway of Death, those in the middle may abandon their vehicles, but either way, you then kill all the stuff in the middle. Don't worry about all those "ants" running out into the desert. They're mostly city boys and the desert will take care of them.