I don't think the Z-film was altered other than possibly removing frames.
I think what happened at Z-313 is hard to understand because of all the misinformation from the autopsy and the WC.
If we knew where the shots were coming from and when I suspect Z_313 would make a lot more sense than it does.
IMO
Clint Hill has said many times that he saw a small bullet hole in the hair line just forward of JFK's right ear ( the exact spot where the "explosion" is shown at Z 313.)
If that explosion had occurred at that spot, Hill could not have see a small bullet hole there. (Because there would have been a huge hole there) Hill pointed to his right temple and said that he had seen a bullet hole there and the back of JFK's head was gone....