Just thinking out loud here ...
The fact that the sixth floor of the TSBD was conveniently empty by pure happenstance at the time of the JFKA seems to slide past everyone. Because it was in fact empty, CTers focus on how the shooter(s) got in and out of the TSBD while LNers focus on how Oswald did the same.
But what if it had not been empty? More to the point, how could anyone possibly know it would be empty?
What if Jarman, Norman and Williams had decided to watch from the sixth floor instead of the fifth? What if members of the floor-laying crew (which included Williams and sometimes Norman) had decided to remain and watch from this perch or get their lunches and return to it? What if other TSBD employees had decided to watch from this higher vantage point rather than the second, third, fourth or fifth floors? They conveniently didn’t – but how could anyone possibly know they wouldn’t?
As far as I know, the sixth floor would have been accessible by stairs or elevator throughout the JFKA. No one took the obvious step of placing a sign or two saying "Men Working – Sixth Floor Temporarily Closed.” (No, the power did not go out in the TSBD. Geneva Hines was talking about the bank of extension lights on her telephone.)
The fact that the sixth floor was empty seems to me to be pure happenstance that could not possibly have been anticipated with anything like certainty. Isn’t this pretty much the kiss of death for any conspiracy involving someone other than Oswald as the shooter on the sixth floor? It would have been insanely risky, would it not, to assume that not only could the shooter enter and exit the TSBD without being observed but that the preparations and actual shooting would not be observed because the sixth floor would be completely empty?
Turning to Oswald, even he is somewhat problematical. Would he have simply shot his fellow TSBD employees like Jarman, Williams and Norman? I tend to think not. Would he have attempted to escape, holding them at bay with the rifle until he reached the stairs? Nah, to have any hope of leaving the TSBD, he needed to be unobserved. Would he have shot JFK and then shot himself? Possibly, but I doubt it. Would he have called off the JKFA entirely? Possibly, but I doubt it. My guess is, he would’ve shot JFK and allowed himself to be arrested since I don’t think he could have had any realistic expectation of exiting the TSBD in the first place.
Oswald had to have factored into his thinking from the get-go the possibility that there would be others on the sixth floor; indeed, his creation of his sniper’s test behind stacked boxes suggests this was exactly what he was thinking.
Yes, he did shoot Tippit - but by that time he had astonished himself by escaping the TSBD and now had a realistic hope of escaping completely. I doubt he had any such hope when he decided that morning to carry out the JFKA or when he was sitting in the sniper's nest.
The distinct possibility that the sixth floor would not be empty, and the fact that it was pure happenstance that it was empty, seem to me to be possible to reconcile with Oswald as the shooter but impossible to reconcile with any conspiracy involving someone other than Oswald as the sixth-floor shooter.