If JFK turned his head at Z-143 in response to Oswald's first, missing-everything, shot, would you expect there to be "definitive proof" of it?

JFK evidently he didn't recognize the sound as a rifle shot.

He wasn't the only person in the limo who didn't recognize it as a rifle shot.

Secret Service Agent Roy Kellerman said it sounded like a firecracker.

Regarding JFK's waving to the people on his right, maybe he was just returning their waves. Maybe they were smiling at him and cheering, so he figured, "It must have been just another firecracker, or a motorcycle backfire."

Good point.

I assume that by "the first shot," you mean your first shot at Z-145 or whatever?

Jack and Jackie didn't exactly jump out of the limo and start flopping around on the grass, but they did turn their heads at Z-143 -- Jack quickly, and Jackie in an accelerating manner.

Jack Ready, Glen Bennett, and George Hickey in the Secret Service follow-up car all consciously reacted between Z-140 and Z-150.

Conscious reactions to the sounds of an unexpected nearby gunshot typically occur about a second after the shot.

Between Z-133 and Z-143, JFK is looking at the spectators to his right (and either waving to them or brushing his hair back), as is JBC, who is just looking at them.

In Z-143, JFK starts turning his head quickly towards Jackie and then turns it quickly back to his right, towards the spectators.

At Z-150, JBC quickly turns his head to his left about 45 degrees and is "captured" looking almost straight ahead in the Z-161 Robert Croft photo.

In Z-164, JBC starts turning his head quickly back to his right about 120 degrees to try to catch a glimpse of JFK over his right shoulder, but he can't "see" (i.e., recognize) him because by Z-180 JFK has turned his head far to his right and has raised his hand and forearm to wave to someone, thereby partially obscuring JBC's (peripheral) view of his face.

None of the things you mentioned, other than the JBC Z164 turn, are definitive signs of a reaction to a gunshot. There are other plausible reasons for those movements. The only reason the Z164 is more compelling is because JBC described how he reacted to the sound of the first shot. We see him do exactly what he said he did. If I am correct that the first shot was fired at Z147-148, JBC would have heard it in the Z149-150 window and his reaction would have been roughly 3/4 of a second. The Z155 jiggle/shake of Zapruder's camera is very similar to his reactions at Z227 and Z318, both closely following when known shots were taken. To  me, that is the preonderance of evidence as to when the first shot was fired, even if not proof positive.

Point being: JBC consciously reacted to Oswald's "Z-124" shot about 1.31 seconds after hearing it, not "just over two seconds" after hearing it.

One has to understand how to assess probabilities of independent events. The probability that 25 would independently imagine seeing the same thing, which did not occur, while none would report what actually did occur, is effectively zero.

So are you NOW claiming that these 2 women sprinting up the sidewalk are in the Wiegman Film? That we just can Not see them? Is that your new wrinkle? This is what happens when you Fail to thoroughly think through your claim. It requires patching. Again, you need to CHANGE THE TIMELINES.

Interesting tidbit on LHO first crossing the border into Russia:

"There is also a curious snippet from a man named Bill Trousdale, who happened to share a train from Helsinki into Russia with LHO. Fellow Americans, the pair bantered a bit on the train.

According to JFK Facts, “Trousdale saw Oswald get special treatment from the Russian border guards: “At the border my bags were given pretty thorough going over,” Trousdale wrote, “but they scarcely looked at Lee’s.’”

Was LHO already a de facto KGB asset, and waved through the Russian border?

Some have posited LHO may have been working for CIA'er Bruce Solie, who is further posited to have been a KGB asset.
That is an interesting CT on JFKA-world.

So...how long did LHO remain a KGB asset? Was LHO a witting KGB asset? Did the KGB "lay off" LHO onto G2 or DGI??

Why was LBJ so concerned about LHO-Havana connections?

https://www.scholarlyroadsideservice.com/blog_entry/travels-with-harvey
Travels with Harvey - scholarlyroadsideservice.com
Feb 15, 2018 · But that evening Alexis and I went on line and confirmed most of the details in stories about Oswald’s travels. All that is left is to see Bill’s diary itself. Roll the clock forward three months …