I just looked at the HoldenMalden mission and I followed the waypoints of the playersgroup. (named nase)
They start with in default behaviour/speed/combatmode only the formation is line instead of wedge. allowfleeing 0 in the init
When they pull back behaviour changes to stealth and allowfleeing become 1. That's all they did. The trigger that execute the retreat is named Zdrhejte!!! (mapgrid Cj75)
But when they flee the tanks are still far away (they just enter a trigger area 200m away from you) and your group doesn't really take fire from bullets or shells, only a few when they hesitate. Most members run like hell.
I think AI will flee faster from tanks because they know they don't take a chance against them.
Now for your convoy situation...
All the retreat scripts I know about need waypoints (Rush-retreat by GB and Fighting retreat by Bremmer)
You are not using waypoints, but you can sort of create them with gamelogics or markers and domove commands...but I don't think that the problem.
When they are under fire in an ambush,danger is pretty close and they need to defend themself first before they can flee. The only way I know about to make them run without fighting back is
"_x allowfleeing 1" foreach units yourgroup;"_x setunitpos ""UP""" foreach units yourgroup; yourgroup setbehaviour "stealth";yourgroup setcombatmode "blue";yourgroup setspeedmode "FULL"
Thing is, how many will survive?
The main problem imo are the vehicles in the convoy. When they got ambushed the drivers drive like drunks, collide with eachother, your men disembark and they pile up behind the vehicle, etc...
I don't think you can compare the holdMalden mission with a convoy situation.
Just my 2 cents and good luck