i cant see why you couldnt fill it from tailshaft hole as you use that to drain it, check gearbox specs to find out how much oil you need, im picking itll be a slow processes like that and youll probably need the gearbox to be out of the car upsidedown to get enough oil in

i was also thinking as these plugs are so chunky you could probably drill a fair size hole through it sideways then shove something in it and turn that

have you tried vice grips they usually get stuff like that after a bit of mucking around, also try tightening the bolt a little more then try undoing it as this sometimes loosens it up a little