The "French" version was created using a frame interpolation program, imo. The frame, call it 312a, that appears after 312 is a separate, independent and complete frame not a result of video aberrations. JFK's head moves to a halfway position between 312/313. So does the limo. Splitting the distance the limo went in one frame(312-313) into 312/312a/313 cut the limo speed in half for those two frames and creates the impression of a slowing limo.
If you click the video and save as a"Watch later" under you own Youtube account you can adjust the settings to double the speed of the video which will be much closer to reality and the faked limo stop/slowing becomes obvious.
Frame 350 has some artifacts that are probably a result of experimentation with the I.P. program. The curb in the French version of frame 350 looks to be from Z frame 349. Two indicators are Altgens missing shadow across the curb and the location of the yellow curb near Greer's side window. Both match Z frame 349. The yellow curb remains in the same location from 349 to 350. Then it jumps forward and catches up by frame 351. Being that the addition of a frame 312a is obvious it is not a big surprise to see artifacts of other interpolation attempts. Another clue in frame 350 is the vertical blurring of the curb in the French copy but not the real Z 350. The bright horizontal curb almost doubles its vertical size from frame to frame due to a slight vertical camera jiggle. Compare Z frames 349 and 350 for an example. 349 is blurred and so the curb is much thicker than 350. But in the French version 350 is thick and matches Z frame 349. There also several other small blemishes on the curb and discolorations in the grass just above the curb that also, I think, prove the French frame 350 has elements morphed from frame 349.