With respect to, "we don't see the back of his head being Blown Out in the Zapruder film..........", if you look at a high def copy of the JFK Limo back seat area, we DO SEE a Large Basketball Size Blood/Brain Matter Stain on the very TOP of the Back Rest. This is the direct result of a Blowout EXIT Wound in the BACK of JFK's Head. This Stain also explains DPD Motorcycle Officer Hargis being HIT on the (L) rear of the JFK Limo with Blood/Brain Matter as well as whatever Jackie was after as she crawled across the top the Limo trunk lid. Solely relying on what you see on the Zapruder Film to reach a definitive conclusion is a Trap.
Not necessarily. In the film, we see that Kennedy's head suddenly jerks
forward (due to the impact from behind) and the blood and brain tissue is expelled
upwards/forwards. We also know that there was a slight wind which carried this material backwards, landing on the back seat area, the rear trunk area and into Hargis.

Also, as Kennedy slumps towards his left, his head passes the particular area that you mention. His cranium was continuing to spurt out blood.