Unlike your example of the bicycle victim above, I think JFK's brains were mashed potatoes. Many conspiracy buff's always mention the "rearward jerk of JFK's head" after the last shot. As a hunter, I've seen the phenomenon of a bullet entering soft tissue and then the bullet imparts its energy to the tissues leaving the exit hole as a jet of soft matter that propels the head or abdomen in the opposite direction of the bullet path. It's a demonstration of Newton's third law of motion in a macabre way.
The Zepruder film, in slow motion, leaves little doubt that JFK's brains were literally "blown out". There was nothing for a surgeon to repair.
Very likely. However, partial lobectomies are done all the time. It was long ago and my memory of that one conversation isn't complete, but I'm fairly certain there was a possibility of saving him with less impairment than you suggest.