Yet Another Shooter??

[Dan O'meara]

That's where Leroy Blevins Sr proposed that the shooter was - just inside the pergola behind Zapruder firing from the 3rd 'window'.
But that window is high and it needed somebody to lift the shooter possibly?
It would account for Zapruder saying the shot came over his shoulder.

Zapruder never says the shot came from over his shoulder. He assumes the shot came from behind him because that's where the police seem to rush to after the shooting.
Marylin Sitzman, stood next to Zapruder, describes the shots as being quiet and coming from the direction of the TSBD building.
Neither of them support a shot being heard directly behind them and the idea that the assassin is being held up by someone else is like something out of a Marx Brothers routine.

[Royell Storing]

  I find it difficult to respect a guy that hid his getting an extra $125K for his film and then presented the facade that he gave ALL the money he received for the film to the Tippit Family. That's 

[Gary Hemod]

Yeah it is a bit fantastical.
I've tried to get Google Earth to get me in the same position and see what the view was like from the 2nd window up with no luck yet.