I believe Jackie was trying to escape, not retrieve any part of JFK's head.
Now, the hydrostatic effects and the 'jetting' of brain matter will rain down behind the vehicle as it moves forward. This is why the motorcycle officers were 'rained' on by brain tissue. Yes, a majority of matter would fall in the inside of the vehicle following the direction of the shot.
Also, the velocity of the 6.5 Carcano is about 2100 fps at the muzzle and would translate into about 1950-2000 at the distance Oswald was shooting.
That is a lot of energy to expend in a soft target.