Neither the autopsy nor any theory of the case that I've ever heard of posulates this scenario. If you're intrested in the known and verifiable facts, here they are:
There are three verified shots. One missed. One hit Kennedy in the upper back/throat. One hit Kennedy in the head.
Gov. and Mrs. Connolly were sitting in what were known at the time as jump seats. These were smaller fold-up seats that were somewhat lower than the rear seat in that model of limousine. They were also somewhat to the center of the car. In other words, if you were able to go back to Dallas on that day, and look directly down at the car from above, you would see that Connolly was sitting in front of and a few inches lower than the president, as well as a little to JFK's left.
There was an entry wound in the upper part of JFK's back, a little to the right of his spine. There was no exit wound in the chest — in fact, no external wound in the chest at all.
There was an exit wound in his throat just under the Adams apple approximately where the knot of the tie would be located. This wound was enlarged by the tracheotomy performed at Parkland.
Although I have never seen a photograph to confirm it (and this is when the car was behind the freeway sign in the Zapruder film), the Warren Commission postulated that JFK had his right hand up, waving to the crowd, and that he was leaning foward when the first shot struck him in the back, exiting his throat, then continuing on to strike Gov. Connolly.
I have seen computer animations on TV which conclusively demonstrate that, if Kennedy had been leaning foward, it would have been a straight trajectory from the sniper's nest in the schoolbook depository to the wound in his back, the wound in his throat, and the wound in Connolly's armpit.
[I should note here that my problem with these animations is that they reveal Kennedy would have had to be leaning quite a bit forward. I am unaware of any photographic evidence or witness testimony that shows this to be the case. When JFK is last seen on the Zapruder film before the car goes behind the freeway sign, he is sitting upright. He is shot in the back/neck as he is behind that sign, and is already reacting when he emerges.]
The last known shot was the fatal one to the president's head. That one is conclusively proven by the Zapruder film to have come from the rear. Why? The Zapruder film shows beyond any shadow of a doubt that the exit wound in the head was at upper right front — roughly an inch or two above and to the front of the right ear. That portion of his head literally explodes. You can see blood and tissue fly straight up as well as forward and downward. The force of that explosion threw JFK's body backward and to his left.
From the known physical evidence, I am left with two questions:
(1) In order for the trajectory of the back/throat shot to line up straight from the building through JFK to Connolly, JFK would had to have been leaning foward in the car at the moment that shot hit him. Why is there no evidence (at least none that I've ever seen) to show that he was, in fact, leaning forward? If he remained upright, as he was when the car went behind the sign from the Zapruder perspective, then how did he get a wound in his back and in his throat?
(2) The shot or shots that hit Kennedy in the back/throat and caused Connolly's wounds did not explode the way the one that his JFK's head did. Why? Was the explosive effect due to use of a different type of round or to the different physics/mass of the head vs. other body areas known to have been hit in the two men?
I believe fully that Oswald was involved in the assassination. Did he act alone? To me, lacking any other evidence (such as might be produced through digitizing the audio), I would need the above questions answered in an honest and believable way before I could reach my own conclusion that he acted alone.
First, the neck shot hit only soft tissue (skin, fat, muscle) It did exactly what it was designed to do as military round, it penetrated. It altered course minimally, which would have been to slightly flatten its trajectory (experimentally proven, which also goes to your question #1; physics explanation available, but its bedtime). The head shot struck hard bone almost directly perpendicular. This overcame the strength of the round (at 2100 feet/sec) and it disintigrated, leaving multiple fragment in the head (see x-rays), and 2 other fragments found in the limo.
But more to the point, the skull is a rigid "box" of a fixed size. As the bullet's shockwave rapidly expands the intracranial tissue (brain) and fluid (cerebrospinal fluid), the skull is unable to accommodate the expansion and (already weakened by fractures) blows out. The soft tissues around the shoulder/base of neck are much more pliable, and will expand and contract rapidly, with essentially no noticible deformity to the naked eye. They will of course be injured by that rapid expansion and contraction, which is how a high-velocity round does much or most of its damage, leaving small entry wound and small exit wound.
Incidentally, the neck shot's shockwave injured (essentially severed) the President's spinal cord at the C6-7 level. We know this because the victim's hands up to the chin with the elbows up and out is a neurologic reflex in response to this injury. (This reflex was described in the medical literature about a hundred years ago.)