And why does JFK's head violently lurch to the rear if he was shot from behind? The documentary "experts" say this is possible but I've never been able to understand why.Two reasons. One is the "jet effect," where the material ejected from the exit wound forces the target toward the shot; the other is that in cases of violent head trauma, the spine goes rigid, forcing the head back.
You forgot to mention this.
Kennedy wore this back brace constantly, and had it on in Dallas that day. It made it difficult to sit for prolonged periods, and certainly the stiffness of the metal stays would have made it like a corset. It would have held him upright. He would have slumped forward in the vehicle, not stiffened backwards, except for the brace.