Googling “Frame 313’ brings up the critical instant from the Zapruder film. There appears to be a ruddy explosion at/on Kennedy’s forehead. If that’s the exit of the bullet taking place, then it answers my question. The entry was from behind. That opens the door for Donahue’s theory, but does not answer whether that agent’s ammunition was finally accounted for. It certainly supports the firer being Oswald as he’s the only one we know for certain who fired any shots.
It wouldn't have been if there were truly a cover up to protect the SS agent.