You believe there is solid evidence the Zapruder film was altered?
Familiarise yourself with Roland Zavada and his report on the authentification of the Zapruder film. You can then put any silly notions of alteration behind you.
The experts - and it wasn't just Zavada although he was, from what I've read, the most knowledgeable person about the film/camera - who studied the film said they found no alteration. So what are we to do with this pretty important conclusion? Simply dismiss it? It's like the backyard photos where experts said they saw no fakery in them. But conspiracy believers reject that conclusion too. Can the experts be wrong? Of course. But you need to show it and not just wave it away.
Garrison subpoenaed the Zapruder film from Time/Life and showed it multiple times during the Clay Shaw trial. Zapruder testified during that showing and vouched for its authenticity. He admitted that he couldn't be sure that every frame from the original was shown or that nothing was changed; but that it "represented" what he filmed/saw that day. Later that film was pirated and distributed by the JFK assassination "underground". It (or a copy of it) was shown on TV in 1970 and then by Geraldo Rivera in 1975. It's from my understanding the one we see today. It seems to me that if the blowout had been altered from the back to the side that Zapruder would have noticed that in the trial? It's a pretty significant change. Did he miss that?
As to the Parkland vs. Bethesda doctor's accounts of the location of the head wound. Why would *emergency* room doctors (remember: not all of them said back of the head; some said side; *see below) at Parkland in a, as they explained, brief hurried setting, be correct about the location of the wound but the autopsy doctors at Bethesda who examined the president for four hours and closely studied the head wrong? What makes the first group right and the second group wrong?
To put it differently: You have one group of doctors whose job it was to save the patient and NOT study the wounds, just keep him alive first, and another group of doctors whose job it was TO study the wounds and not save the patient (since he was, of course, dead). So we have two completely different situations. Which group is more likely to "get it right", locate the wounds properly? Why would anyone rely on the ER doctors over the autopsy doctors? What makes that conclusion correct? After all they missed the back wound and a bullet entrance to the head wound. Why did they miss those two wounds?
Moreover, the doctors at Bethesda have films, x-rays and photos - physical evidence - that supports their explanation. The Parkland doctors have none of that.
It seems to me that if you weigh the Parkland doctors accounts versus the Bethesda doctors accounts PLUS the photos and x-rays and films that it's not close who is right. We can add the accounts of the Connallys and Kellerman who said they were hit/sprayed with blood and brain matter. How could a blowout in the back of JFK's head deposit matter to the front of him? It's impossible. The motorcycle officers - Hargis - said he "rode" through it as it came down. He also said he was watching JFK and saw no exit wound out of the back of the head; only a "splash" out the side. So why didn't he see this rear blowout?
Add to it Zavada's analysis and the timeline and chain of possession of the film and I don't see how one can conclude it was altered.
*Dr. Charles Baxter, one of the attending physicians at Parkland: After JFK was pronounced dead "We had an opportunity to look at his head wound then and saw that the damage was beyond hope, that is, in a word-- literally the right side of his head had been blown off." Right side of the head NOT the back. Yes, other doctors said back of the head; why are they right and Baxter wrong?
Source/link for the above from Baxter:
https://www.jfk-assassination.net/russ/testimony/baxter.htmJFK's skull x-rays. On the left in 1960 and on the right after the assassination/during the autopsy. No back of the head/rear exit. It's intact. Just as the Bethesda doctors said and the Zapruder film shows.