Looking through all the films and documentation, crammed with contradicting evidence I get dizzy and frustrated. I can find myself convinced by an LN argument then convinced by a CT counter argument. It goes on and on around in circles.
Then I realised that the explanation that solves the whole riddle is that there were two totally independent plots, the participants of which, had no idea that the other was in play (until the shots rang out that is). I think this subject has already been discussed here, so I searched a little but the search motor came up with nothing. Please point me to existing thread and/or books which relate to this theory. Anyway, how I see it:
There were two unrelated plots (a theory):
1. The conspiracy involving the recruitment of one or two professional gunmen (not a wide "government" plot but a small group of powerful individuals),
2. Lee Harvey Oswald, the "lone gunman" (playing out a plan in cahoots with the CIA).
On the day of the assassination, as the ground level sniper(s) prepared to take the kill shot, they heard a shot ring out, and another. WTF? Was that a backfire? A firework? Another shooter? If there were two professionals down there, they might have thought that the other had already let off a shot. Whatever, I have to to take my contracted kill shot (from the front, side or wherever).
As for Oswald, he had also seen the route plan and realised that it passed by his workplace. What an opportunity to create a scare and reinforce my image as a pro-Castrist! Yes, Oswald's intention was to merely let off a few rounds (with his rusty, misaligned rifle), as if it was an assassination attempt. He fired into the ground with the first shot. Another went further along, hitting a kerb stone and spitting out fragments. A third shot? Maybe, and maybe it did end up within the presidential limo. He heard shots that weren't his! Or was that an echo?
He didn't see that JFK had been hit and he descended, thinking only that his little scaremongery exercise had gone as planned. Of course, the rifle and bag would be found but that was part of the plan - a little jail sentence for disturbance of the peace and his portrayal as a communist/pro-Castrist was nicely bolstered, ready for the next part of his mission (as an inside informer for the CIA in Cuba).
Or perhaps he did see Kennedy's head explode and was thrown into a panic, so he grabbed his revolver from home and decided to leave town. His flight was thwarted by Tippit. He dashed into a cinema and tried to find someone who could give him a lift out of town. He's captured and realises the dilemma he has found himself in. Did he feel that he had been set up? "I'm just the patsy!". He tries to get a phone call out to Langley for help...
Then he was at the mercy of the bumbling DPD, the FBI and finally the Warren Commission to tidy things up for the American public and the World stage, to make it all point to Oswald, at all cost.
So in fact on that fateful day, there was an incredible coincidence. That's what it was. Two separate plots, totally unrelated and it is that which has led to decades of all these half-convincing theories.