Welcome back Ernal.

I know the frustrating feeling of mangled bolt heads. This is a pretty drastic idea but it has worked for me in the past. Have you got a wrench with some grippy teeth on it? I have one and it does wonders when trying to remove these worn bolts. Sure it will mangle it even further but you should be able to get it out then get a new plug.

Having to change oil thru the back of the gearbox would be a pain in the ass, so I would try really hard to get this plug out (but Im sure you already have).

Im sure someone else will have an idea.