Just to recap.....The thunderbird has been a 4 door but the Mustang has never been in 60 years. It would be a better candidate for this platform, also it wouldn't break any lore. Your argument doesn't hold water.
The Mustang has always been a 2-door coupe. Anything else isn't a mustang.
'M-Series' or 'AMG' or 'Raptor' is a performance level and can be applied to different models. You could have a Raptor Mustang but its not practical but still needs to be a 2 door.
Power and transmission options...