When you look at JFK BEFORE he is hidden behind the Stemmon's sign, he has his right hand raised almost to the top of his head. When he emerges from behind the sign he has quickly lowered his right hand towards his throat and quickly raised his left hand up from his lap towards his throat.

During that same interval, Connally is looking to his right at frame 193, frame 200 and frame 224. Connally does NOT appear to make any reaction to a shot until (starting at) frame 224.

SUMMARY:
1. Kennedy reacts to a shot (probably to his neck) between frames 200 and frame 224. Because he has to move his arms so far, it had to have been several tenths of a second before frame 224. It looks like it was closer to at least a second for him to react like he did.
2. Connally makes no reaction (especially NOT a serious wound) until AFTER he has emerged from behind the sign
3. Because JFK moves both his hands towards his throat, it is reasonable to assume that JFK's first wound was to his throat.
4, A reasonable conclusion based on the reactions that can be seen in the Zapruder film is that
a. First JFK was wounded in the neck
b. The next shot (about a second after the first that hit JFK) wounded Connally for the first time.
5. If thwo shots really are closer together than about 3 seconds, it suggests that there were multiple shooters and that means a conspiracy killed JFK.
You make a good point about Kennedy?s reaction relative to Connally?s. When he emerges from behind the sign at Z225 he?s markedly distressed and his arms shoot up toward. However it?s physiologically impossible (or at least unlikely) for him to have been voluntarily reaching for his throat after a gunshot wound to his neck due to disruption of the spinal cord. More likely, a perforating shot from the rear struck the two men at Z224ish, and the sudden impact to the trapezius chased his already ascending arms to rocket higher.
Various analyses have found evidence of JFK reacting to something (supposed by the HSCA To be a gunshot) at around Z190. Several witnesses reported a bullet strike the pavement near the limo and seeing JFK flinch afterwards. This aligns with a rugged tear in the rear of the scalp (usually thought to be the entry point of the fatal bullet) and a corresponding circular metallic fragment embedded on the exterior of the skull.
TL;DR.- What I suppose is that a missed shot fired near Z180 hit the curb and sent shrapnel flying to hit JFK, causing him to exclaim (per Kellerman) ?My God, I?m Hit!? Then at Z224 both men were struck.