So, David Von Pein, you think it's preposterous that the conspirators would plan to shoot at Kennedy from all directions and blame it all on a single lone gunman shooting from the rear? But, the fact is that the shooting didn't go well. The first shot did come from the rear, and they must have expected that it would have hit Kennedy in the head, and then it would have been all over, and the other shooters would have been called off. So, the killing of Kennedy did not go well for them. But, they probably thought ahead of time that they could always claim that an entrance wound was an exit wound- which is exactly what happened.
But, while we are discussing implausible expectations, why would Oswald assume that he would have the 6th floor all to himself? He knew there were a lot of workers up there, so why couldn't one or more of them have stayed and eaten lunch there? Didn't Bonnie Ray Williams eat his fried chicken there? Weren't the bones found? That was shortly before the assassination but it could just as easily have been a little later. And by then, he could have decided to watch the festivities from there. Didn't he and several others watch the festivities from the 5th floor window directly below? Weren't Victoria Adams and Sandra Styles watching from the 4th floor window? Didn't Geneva Hine say that she watched from a window? So, according to you, this was Oswald's thought process:
"I think I'll shoot Kennedy from the 6th floor even though I have no way of knowing if it will be empty OR if it is empty whether it will stay empty or if someone will burst in on me. And even though a crew of men are working there and one or more might decide to stay and eat there, and even though one or more persons might decide to go there to watch the motorcade from the window, and even though there is absolutely nothing I can do to secure that space for my vile task, I am just going to dismiss all that and risk it. I'll just take a chance and hope for the best."
You haven't even established a motive for Oswald wanting to kill Kennedy, never mind going about it so wantonly and recklessly and stupidly.