(although we really can't tell when JFK began reacting)
Yes, we can
You are guessing that he is not already reacting behind the sign. You are guessing that his right hand suddenly moved from a high wave at z193 to a knarled clutch at z223 for no reason relating to being shot. The fact is that we cannot see him when he is behind the sign. So if you think he is not reacting behind the sign it is not based on something that can be seen in the film.
He said he was hit in the back a perceptible time after that
He said he was hit a "split second" after hearing the shot. He said the two events were so close together he thought there was multiple people firing at the same time or that an automatic rifle (which can fire two rounds in a split second) was being used.
His memory of the event has the typical temporal distortions experienced by those who go through a traumatic, life-threatening event. That is to say, in his memory of the event time slowed down in a very radical way. The shot he heard and the shot he felt were the same shot.
He also said it was enough time for him to recognize it as a rifle shot and turn to look back over his right shoulder in a serious attempt to see JFK and then decide to turn back before he was hit in the back. He estimated it to be two seconds. His words, not mine.
The right side of his jacket didn't burst forward because JBC was concerned about JFK.
I agree. But I also see no evidence that the jacket did burst forward. Neither did the WC or HSCA. Where do you see JBC looking down at his chest? He said he immediately saw blood and knew that he had been shot.
It burst forward as a result of the bullet exiting his chest.
Again, that’s your guess. It is not my guess. My guess is that it is consistent with jacket movement due to his right arm moving as he prepares to turn around to see JFK.