It's a lot harder to frame somebody for a phony bus ride and cab ride than it is to make a phony movie. A phony bus ride and phony cab ride require phony witnesses; a phony movie does not. A phony bus ride and phony cab ride have to displace the real thing that happened, the real thing that Oswald did, which could have its own set of witnesses and evidence that conflicts with the phony story. But, if you alter the film and add the content that you want (which was Lovelady sitting at the desk) you just eliminate the original so that the phony one is all there is; there's no alternate. And last but not least, if the whole idea of falsifying a movie is beyond the mindset of most researchers, you have even less to worry about.
Millions of JFK researchers believe that the Zapruder film was altered. I'm just asking them to consider that other films were altered as well.