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There is too much in this post to refute point by point. I'll just note this for your consideration (since you highlighted it in bold):

In the current film, Kennedy's head is knocked forward from Z312-313 by the impact of a bullet. No one disputes this. With regard to these frames, Itek noted, "the President's head is subjected to a large acceleration forward." Itek calculated that Kennedy's head is knocked forward 2.3 inches and his right shoulder about 1.1 inches from Z312-313. Bear in mind that each frame represents only 1/18th of a second. But, amazingly, by Z314 the head is suddenly moving backward. I suggest that in the original film the marked forward motion that begins at Z312 did not end at Z313 but continued for at least several frames and probably more, and that this was the forward movement seen and described by witnesses.

* The violent, dramatic backward head snap in Z313-323, which for so many years was thought to be concrete proof of a shot from the front, actually constitutes further evidence of alteration. It has been established that no bullet striking the front of the skull could have caused the backward head snap. However, no bullet striking from behind could have caused this motion either. Warren Commission supporters have put forth two theories to explain how a bullet striking from behind might have caused the head snap, the jet-effect theory and the neuromuscular-reaction theory. Both theories are untenable (see, for example, ("Special Effects in the Zapruder Film: How the Film of the Century was Edited," in James Fetzer, ed., Assassination Science, Chicago: Catfeet Press, 1997, pp. 279-284; Mark North, Act Of Treason, New York: Carroll and Graf, 1991, pp. 383-385). So if neither a bullet from the front nor a bullet from behind could have caused the head snap, what caused it?

The short story: Mr. Griffith finds it difficult to understand how what the film shows happened, so since he cannot understand it, he believes that is proof of alteration of the film.

If you have the time, check out The Zapruder Film is Authentic.