Unless the head was off screen. Weren't you the one who said they didn't want to animate a blown off/blown up head because it was too gory? Regardless, that's probably the real reason we see no head. Also, even at point blank, there would still be some of the head, or at least, neck remaining.
All in all, this argument is becoming one about technicalities/possibilities, and honestly shouldn't have happened in the first place. I don't suppose we can just be content knowing that South used a shotgun on the guy, relieving him of his head, and that's he's dead?
Not quite, you have no clue what part of the helmet that it dude. I'm sorry, but as to whether the head gets destroyed or not, the definition of decapitation is to have one's head removed. IT doesn't specify. If you break your leg, it doesn't matter if it's broken in a thousand places, dislocated and fractured, at the end of the day it is still broken..