There's only one problem with the "back and to the left" argument. JFK did not go back and to the left when the bullet struck him in the back of the head. His head actually moved forward a few inches from frames 312-313. From there he went straight backward. He was already leaning hard to his left when the headshot struck. That gave the false impression that he had gone back and to the left. All one has to do is look forward a half second from frame 313.
At frame 322, JFK is still in the far right side of the limo's back seat. There was no leftward movement from his position at 312.

Another myth bites the dust.