The eyewitnesses state JFK reacted to being stuck by the first shot. You want to believe there was three shots-----prove it.
"Proof" of a fact is determined by the trier of fact - it is whatever evidence that is considered by the trier of fact to be sufficient to establish a fact. You obviously do not consider the abundant evidence of 3 shots to be sufficient to convince you that there were 3 shots. Others do, including all the members of the WC, HSCA. You appear to think that all that evidence is somehow wrong.
JBC's testimony (two shooters?) was struck by the first shot and then JBC was struck "almost simultaneously" by a second shot fired in less than the 2.3 seconds required to operate the carcano?
JBC did not say he was struck almost simultaneously with a second shot. He said he heard the first shot, realized it was a rifle shot and turned around to try to see JFK but could not as he had moved. As he turned back to turn to his left he said he felt being struck forcefully in the back by another shot. He says that as he lay back onto his wife he heard a third shot and felt the spray of brain matter over him. Even though he did not hear three shots, he observed three shots.
Explain the wound in JBC's back if the bullet does not first pass through JFK.
It is certainly possible that JBC's back wound was caused by a bullet that had not passed through JFK. In fact, one can make a reasonable case that in order for the bullet to have missed his right lung as it did, it had to have been travelling at a very small angle to the direction of the car - almost from directly behind. That means it struck him when the car was farther down Elm when the angle to the SN was very small.