By "forward" I mean in the direction the car is moving.
Really? So everybody is "sailing forward" because they're all moving in the same direction as the limo?
Brilliant stuff.
This is somehow supposed to be refuting the unequivocal evidence of no shot at z271?
You wrote for your first big point:
"What causes him to move forward from z272 - z280 before he falls back onto Nellie?"Look at this clip Jerry posted. In it Nellie appears to move from one side of the metal strut to the other. It's not because she's moving,
it's because the relative positions of everything in the limo is changing as the limo approaches Zapruder's position. This is not an optical illusion, it's a really basic understanding of how positions change with a changing perspective:
There is no "sailing forward" or "sailing backwards".
JBC crumples to his right after being shot, he straightens up slightly and falls back into Nellie's lap.
That's it.
How you imagine this is unusual or somehow contradicts the clear evidence that no shot occurs at 271 is in your mind only.
So let's move on to the next big point that needs explaining:
"2. why we see the hand movement between z271-272. What causes the appearance of the hand and hat to change between z271 and z272?"I made this Gif to highlight what nonsense you are talking. It comprises of z271 and z272:
Z271 is slightly blurred while z272 is much clearer.
This is what is causing "the appearance of the hand and hat to change".
In fact, there is no change as we can see. And remember, this is the moment the bullet is exploding out of JBC's chest and shattering his wrist. Clear evidence no such shot is taking place.