Why do you find it hard to believe that JBC has already been hit in Z-230?
In which Zapruder frame do you think JBC was hit?
The evidence suggests he was hit by two bullets.
The evidence shows that the missile that struck his thigh and did this:

was likely not the same missile that did this:

The position of JBC at the time of the third shot at z313 and the trajectory of his wounds (along the direction of the right femur and from right armpit to below right nipple) eliminates JBC being hit that shot. So I conclude that he was hit on the first two shots.
The evidence establishes that there was a shorter space between the second and third shots and a longer pause after the first. The evidence also establishes that the first shot was after z186. It is difficult to pinpoint but I put it around z193, which is just as JFK clears the last part of the oak tree branch as seen from the SN. At that point, the right to left trajectory from the SN through JFK's midline goes on to the left side of JBC, nowhere near his right armpit.
The shot pattern puts the second shot a perceptible amount of time after the midpoint (313+193)/2=z253. This means it was after Altgens' #6 photo taken at z254-255. Hickey is still turned around looking rearward and he said he was turned forward looking at JFK at the time of the last two shots. Greer said he turned around immediately after (almost simultaneously) the second shot. He begins that turn just before z281. There is also distinct forward motion of JBC beginning at z271-272. Hickey also said that he saw the hair on the right side of JFK's head fly forward at the time of the second shot. That hair flies forward at z273-276. So I put the second shot - the one that struck JBC in the right armpit, exited under his right nipple then struck the back of his right radius which is positioned directly in the exit path - at z271-272. That is the shot on which Tague said he was struck. That is also the shot that Greer said gave a concussion effect as if it struck something in the car. His right ear was a few inches from the damaged windshield frame.
So, to answer your first question,
I do agree that JBC had been shot by z230. But only in the thigh.