With regard to the head shot they, again on the computer animation, showed the exact position JFK was at when the bullet hit. They declared that medical records proved that the entry wound was at the back of the head quite high up, this taken with the computer generated position proved that LHO had a direct shot from his window and that it was impossible for the bullet to be fired from anywhere to the side of the car.
They also claimed that even though he was shot from behind there are many documented cases which prove that it was not unusual for his head to be flung backward from the impact of this particular entry point. As to the exit wound, I'm not sure, but if he was hit high in the head then it may have just taken the top of his head off without affecting his face. Are there any medical records supporting or disproving this? As mentioned before I'm no expert on this.
btw the Zapruder film was one of the main sources for the computer animation.