I am of the firm belief that the Zapruder film is unaltered, certainly in any meaningful way. One apparent 'proof' of alteration are the so-called inconsistencies between the Zapruder and Nix films. This is dealt with in the following article by Millicent Cranor at the Who.What.Why website
https://whowhatwhy.org/2018/07/12/jfk-assassination-film-proof-of-tampering/The inconsistency in question revolves around the following frames:
In the Zapruder frame above there appears to be a sizeable gap between Jackie Kennedy and Clint Hill. In the Nix frame below their heads are more or less touching. How could this possibly be?
The only mystery here is how this easily explained 'illusion' ever gained any traction. It is caused by Jackie leaning diagonally across the trunk. I tried to write down what was happening but found it easier to make my own vid. The upturned baking tray represents the trunk of the limo, the spoon represents Jackie, the lighter represents Clint Hill (the point of this video is just to show the 'illusion').
It starts off with an overview then moves to the left (Nix position) where we see the spoon appears to be touching the lighter. It then moves to the right (Zapruder position) where it reveals a sizeable gap between the two items:
That the Nix and Zapruder films are perfectly synchronised is ably demonstrated in this following Gif
The original Ant Davison clip: