How does that image, you referenced above, possibly provide a description of the momentum physics that would still cause is head to snap back? The mv-momentum transfer of the bullet to his head as shown above would have snapped his head foward.
That illustration only confirms that the momentum is vectored to the right and down. His head violently snaps back and up in the opposite direction.
Show me the physics, forces, mass motion, vectors, dynamics that support the theory that the shot came from the back and would cause his head to snap back.
You mentioned "jetting" of the contents to snap his head backwards, yet I have never heard of this explanation before. Do you have a source? A computer modelling of the movie would answer that fairly quickly. I've shot many targets in my life, including plastic gallon jugs of water, clay pots of water, etc., and never once has one of these targets come back toward me, even though there is a huge jet of water out the back. They always absorb some of the momentum of the bullet and go away from the source.
By the way, Nut, (you don't mind me calling you "nut", either, do you?) I'm still open on this issue, and that is the motivation for my questions. If a theory is to be accepted, it should be able to withstand the scrutiny of all questions. I still have questions left unanswered.