I still don't know if a 15 sec delay would work. Could run into problems...
You do realise that is already how it works right? Exiting to the main menu from combat results in a 15 sec timer with the question "are you sure you want to go to the main menu?"
If there was a change to the existing "legal CL" mechanics just for Multicrew, that would be astonishing.
Furthermore, anything that works for Multicrew works doubly well for CQC, since an ingame CQC lobby would be more popular than multicrew.
I predict multicrew will be a short lived gimmick that will become quite unpopular once people figure out how to troll each other with it, intentionally or unintentionally. Would love to be proved wrong though