There’s no reason to believe that it isn’t Zapruder. The camera strap is the giveaway clue.
Sitzman said she talked with a G-Man there at the top of the knoll. Could be him inside that shelter. Hester was wearing a Long Tie, but I think the gent we are seeing inside the Shelter is much stockier than the thin Mr. Hester. There's a reason Towner 3 has the guy/kid on the fence, and the image inside the Shelter Cropped Out 90% of the time. (1) That picket fence location was occupied during the assassination, and (2) there was supposed to be NO G-Men on foot inside Dealey Plaza at the time of the assassination.