I don’t remember seeing that movie, but I think that I understand the concept. Do you understand that it is entirely possible that BRW and Givens are telling the truth? That it is not impossible for them to have just missed each other on the sixth floor and that LHO simply had to stay hidden and quiet while BRW was on that floor?
In this scenario the workers on the 6th floor, including Williams and Givens, have an elevator race down to the first floor and most of them hear or see Oswald still working on the 5th or 6th floor. While Williams collects his lunch Givens realises he's left his cigarettes on the 6th floor so goes back up where he sees Oswald, apparently still working. As Givens heads back down Williams is heading up, it is during this time that Oswald takes up his position in the SN and waits there quietly while Williams has his lunch.
Would you agree with this assessment?