I'm going to have to disagree with you, as I do believe that Muchmore's film and Zapruder's film show the EXACT same headshot, and indeed the EXACT same scene - obviously taking in to account the different perspectives of the photographers.
I can see NO inconsistencies.
I can see 100% correlation between things like:
* JFK's head being positioned forward and to the left
* Jackie's right elbow immediately behind JFK's left shoulder
* Moorman's position in the background
* Kellerman's head pointed straight ahead
As far as your "if Muchmore was the only thing you had to go on, you wouldn't even know that the guy was shot" comment... seriously ? There's clearly a halo of blood and brain matter ejecting from JFK's head just as is seen in Zapruder.
I think a reasonable person can conclude that both photo's are depicting the EXACT same event and the EXACT same instant.
They are obviously depicting the same event but the question is whether the head shot occurred at the EXACT same instant. You can't make that determination by eye-balling the films. You need to know the exact frame rates for both films then match them up.
For example, there would be differences if frames were removed from the Z-film intended to speed up the limo. This might be detected by synchronizing both flims at frame 313 of the Z film, then advancing the films 1 frame at a time and trying to match up landmarks common to both films. Problem being, an offset of a few frames represents at most a few feet depending the speed of the limo and the camera operator's panning rate. IOW, it is extremely difficult to detect the EXACT frame when films do not match unless enough frames have been removed from 1 of the films. Also, we have no way of knowing whether the films were edited to match each other.
For what it is worth, here is an analysis of the limo speed for the Zapruder and Nix films on a frame by frame basis. Limo speeds are relative to photo units and actual speeds can be calculated by using physical rulers in each film (i.e. tire diameter) to establish a pixels/foot ruler.