The interface issue was fixed in recent beta patch hopefully. Try asking Bobb4 how they run a coop mini campaign
http://forum.1cpublishing.eu/showthread.php?t=26483
If you need some quick 2 vs. 1 training I think it will be quicker to spawn aircraft simultaneously in 3 spawnplaces with airstart than to wait till a coop mission reloads every time. Just type "GO" in chat before spawn to spawn simultaneously. You can place several dozen spawnpoints on a map and chose different ones depending on the kind of training mission you want to fly.