As Kennedy emerges from behind the sign, the involuntary reaction has yet to show itself. We can see his right hand and arm, that he had been lowering after waving to the crowd, still dropping. He did not begin reacting while behind the Stemmons sign.
You are still speculating that the lowering of his hand was not part of the reaction in itself. We don't know that.
I would point out that the WC thought at z225 JFK was reacting to being shot (WR 98):
"When President Kennedy again came fully into view in the Zapruder film at frame 225, he seemed to be reacting to his neck wound by raising his hands to his throat. ... According to Shaneyfelt the reaction was ?clearly apparent in 226 and barely apparent in 225.?
The HSCA found that JFK was reacting well before z224 (HSCA Rep. 46):
"By Zapruder frame 207, when President Kennedy is seen going behind a sign that obstructed Zapruder's view, he appears to be reacting to a severe external stimulus. This reaction is first indicated in the vicinity of frame 200 of the Zapruder film. The President's right hand freezes in the midst of a waving motion, followed by a rapid leftward movement of his head. (43) There is, therefore, photographic evidence of a shot striking the President by this time."
In the footnote on page 44 of its report, the HSCA noted:
"In its report, the committee's photographic Connally reacted to his wounds evidence panel suggested that Governor interval might have been approximately one second after President Kennedy. This interval might have been even less, but a sign obstructing Zapruder's field of view made it impossible to study the Governor immediately after the President first appeared to be reacting to having been shot."
So people who do not share your confidence that JFK is not reacting behind the sign are not being unreasonable. They are just unpersuaded by the evidence and your argument.
The reactions of both Kennedy and Connally begin at the same time. They are in sync.
Only if you are correct that JFK was not already reacting behind the sign. Many, many witnesses said he brought his hands down toward his neck/chest in response to the first shot. They could see JFK "behind" the sign.
In any event, what is the significance of them both beginning to react at the same time? That does not mean that both were reacting to being hit by it. JBC said he reacted to the first shot, which Nellie and about 20 others saw that JFK also reacted to.