umm.. I don't know if you really need scripting for this.. :-\
I'd say you just make a radio trigger with on activation field holding this line
convoy=true
then you make waypoints for the leader unit of that convoy.
set the first waypoint right on top of that leader unit and put in the condition field this line
convoy
that'll make it so that the convoy won't get moving untill the condition "convoy" is met. and it won't be met untill the radio trigger is activated.
for the second thing you'd want to you'd just want to set them in captive mode.
type this line in the init field of every unit in that circling convoy and they won't get shot by the enemy
this setcaptive true
Sorry if I misunderstood what you mean, but that's how I would do it.