I guess that if one team sucessfully managed to assassinate the president and another team to manipulate the autopsy as well as the whole chain of evidence then an additional team wouldn't have insurmountable difficulties to alter a number of film frames...
Oh I think it's very possible and likely that the film was altered. Possibly removing a short limo stop and a hole in the back of his head and the debris that went backwards. Again that's just a possibility and a personal opinion.
I mentioned several theories and I'm not sure which one you adhere to. But someone posited the idea that they placed multiple images of the shooter in the film to give a hint about the conspiracy. That's a theory I don't accept.
I have been learning elements of photogrammetry in a piecemeal way but it has allowed me to debunk a couple dozen claims that the Zapruder film shows multiple alterations. Still I think there are a couple issues that have great Merit. I am still a conspiracy theorist.