JohnM
A good reason For Jackie wanting to get out of the limo is that one moment she is cruising along in the Dallas sunshine, then seconds later she is staring at her husband's exposed brain:
At interesting point about the above Z-frame close-up is JFK's hair. It's there on the back of his head but ends abruptly in a straight line extending from the top of his ear.
There is no hair forward of this straight line.
It must surely be the case the large piece of scalp on the right side of his head has been blown over to the side (the injury described by Jenkins). It is possible to make out a large piece of skull bone, stuck to the inside of the scalp.
Obviously, the Tinfoil Brigade won't be able to see this straight line where the hair seems to end abruptly.