1.) Who said there were three shots? What if there were only two?
2.) The problems with a shallow back wound: a) Ruby shot Oswald with a snub-nose .38 special revolver. It's not a particularly penetrating projectile. Still, the bullet went from one side of Oswald to the other. A rifle bullet at any normal velocity would go at least as far. b)According to the Edgewood tests, the 6.5 Carcano bullets they tested only lost 400fps travelling completely through a simulated human neck made of meat covered with goatskin. Other experiments had determine that it takes a rifle-caliber steel ball about 150fps just to penetrate the skin. So a bullet that only burrows in an inch or so to the body is going to impact JFK travelling on the order of 200-400ft/sec. At 200ft/sec a bullet fired from 200ft away has a 1 second flight time, and will drop 1/2*(32.2ft/s/s)*1s = 16 feet. A bullet travelling at 400ft/sec will fly for 0.5sec and similarly drop 4 feet. So, to hit JFK with suck a pokey projectile, the shooter would have to be aiming for a point several feet above JFK's head, which doesn't make sense. c.) If the bullet was stopped by the spine, it would have caused severe damage to the vertebra.
Your later solution to 1. above seems reasonable.
Regarding 2. above, thank you for the understandable examples. However, since we are working in round numbers let me modify your example. At 400ft/sec, a round will drop 2 ft in a 100 feet distance. Additionally, if it is flying at a 45 degree angle to the horizon, it should only drop 1 foot. This is the difference between a head and back shot. Do you agree?