There is no predicting which way a body will move, or even if it will move when hit by a relitively small calaber round like a .30 cal. Especially in the head where nerve tissues are damaged. It's not like the movies where people fly off the ground.
The "Jet of blood" that came out the back of his head is also normal. You can do it by shooting a ripe watermellon and watch as a spray of juice exits the entry point. It's hydraulics in action.
The final head shot by Oswald was from a distance of only 88 yards. I have stood on the 6th floor of that building and looked out. It would have been an easy shot, even for someone who wasn't a Marine Corps marksman.
Perhaps the force of the bullet sent JFK's body forward: JFK's rso banged forward against the corset, then jerked back from the force of banging against the corset that didn't "give".
>>I thought Kennedy moved backwards after he was shot from behind<<
Probably stiffened from the pain. A commom reaction.