The fact that you have to put several "ifs" into your scenario means that nothing you are saying is an established fact. It's simply your guesses.
The Robert Hughes film shows something white moving in the Sniper's Nest window as the limo turns onto Elm Street. Oswald was wearing a white t-shirt that day, so it's reasonable to assume that what we're seeing is Oswald getting ready to fire his first (missing everything) shot.
IF Oswald fired it half-a-second before Zapruder resumed filming at Z-133 (as is indicated by the nearly simultaneous, rapid-but-non-startle head movements of everyone in the limo but Greer), the limo would have been at a sharp downward angle to him, and this sharp downward angle may have necessitated the short-rifle's muzzle's being inside the building when he fired it.
