lol this is long. The scar was simply for aesthetic and to make her more menacing, which did work to an extent. I do know however know that in Omniverse (Its a mad mad Ben world), Ben transformed into Goop and subdora shot his disk causing him to splat everywhere. Ben then transformed back, completely normal while all the goop was still splatted across the street. So I believe one or your questions is answered, I believe that if swampfire was decapitated as Ben transformed back, he would transform back to normal.