Your FUBAR scenario with JBC struck at Z270 leaves only 43 frames, or 2.35 seconds, between the second and third shots. That's the bare minimum time the FBI determined the Carcano required to fire two aim shots. That is a theoretical time only.
Not theoretical. That is exactly what witnesses said occurred: “ 'just about as fast as you could expect a man to operate a bolt action rifle” (Emmett Hudson). Most witnesses described the last two shots as being in rapid succession and several estimated the time between them to be no more than two seconds.
You prefer your reasoning. But your reasoning is not evidence. I prefer to base conclusions on evidence. Courts do too.
It's ridiculous to think Oswald would have been trying to fire his weapon that fast. Accuracy was far more important than speed. Why would Oswald even try to fire that rapidly and why should we believe using the bare minimum time to fire his weapon he could have delivered such a deadly accurate shot. The reality is Oswald took 4.9 seconds to aim and fire the third shot.
I would suggest it is ridiculous to suggest that your reasoning should supplant the evidence of 45+ witnesses who described the last two as being in rapid succession, real quick, etc. Besides, reasoning could also work the other way: it makes sense that he would want to get the last shots off quickly as the car was getting farther away.