When you want the convoy to go off the road try changing there behavior on the waypoint to Stealth. They seem to be more willing to drive off road. Also, try giving the vehicle multiple waypoints offroad instead of just one. Likewise, they seem to follow the path. As far as the ambush goes, you would want to give the OPFOR a waypoint where you want them to wait for the ambush. Make that waypoint stealth and hold fire. Then create a trigger and set the activation as BLUFOR. Then synchronize the trigger to the OPFOR stealth and hold fire waypoint. Place the trigger where you would like the BLUFOR to be ambushed. Next you can give the OPFORs next waypoint as combat, seek and destroy to where ever you would like them to go/attack. OPFOR will hold at there stealth waypoint until the trigger is set off by BLUFOR, then they will continue on with the seek and destroy waypoint. Hope this helps.
Not sure if you have seen this in the forums. Tons of info about editing. http://www.armaholic.com/forums.php?m=posts&q=6751