Zapruder’s film did not capture the fence area during the assassination itself since his filming was focused on Kennedy and not the grassy knoll or fence area. The fact he shows the fence after Kennedy’s car was going through the underpass is pretty much irrelevant since any shooter would have been long gone.
None of the films including the Zapruder film prove or disprove a shooter standing behind the fence. There is little evidence one way or the other. We may never know the details of what went down.
We lived in Texas during that time and went through it months after the assassination.
Long gone? Within probably 5 to 7 seconds. Not likely. And Zapruder would have heard the shots from 20 feet to his right at the level of his feet. For someone who was in the Army I do not think so.