There is one way to prove beyond doubt that Oswald could not have been on the 6th floor at time of shooting, and that is Dorothy Garner.
If she is still alive, a simple question could be asked: Did you hear V Adams and Sandra Styles voices on the staircase, when you came out of the 4th floor office?
Mrs Garners statement to Barry Ernest, author of the book "The Girl on the Stairs" was that she did not actually see Adams and Styles when exiting the 4th floor office, but she "heard" them on the staircase.
Given that Adams and Styles did not run into Baker and Truly, or vis versa, means A&S had to have left the 4th floor office by 10 sec post shots, in order to make it to the 1st floor by 60 sec post shots, and exit out an open roll up door by the elevators. Baker and Truly are crossing the 1st floor surely by 65 sec post shots, to reach the rear stairs and elevators by 70 sec post shots, thus would surely have seen A&S arriving any later than 60 sec to the 1st floor.
Mrs Dorothy Garners statement to Barry Ernest was that she followed A&S "almost immediately" and "right behind" them. But she also did not "see" them when she exited the office, so that defacto means Garner delayed at least 15 seconds after A&S exited, which is the time required for A&S to have crossed about 70 ft of floor and go down partway the 1st half of the staircase and be out of LOS of Garner, when she exits the office.
To have missed seeing Oswald on the 4th floor landing however, Mrs Garner would have had to delay approx 43 seconds from when A&S exited the office (they exited at 10 sec post shots), because it would take Oswald at least 50 seconds to reach the 4th floor staircase, and 3 sec more to go down it far enough to be out of LOS of Garner.
If Garner heard "the voices of the girls" on the stairs, that conclusively would prove Garner exited office WELL BEFORE 53 second post shots, because at 53 sec post shots, Adams and Styles have almost reached 1st floor and thats 4 floors of sound separation thus improbable that Garner would have heard their voices.
Barring such a definitive statement from Garner, leaves only the probability of which time is more probable to match best with Dorothy Garners statement of following "immediately" and "right behind" A&S. 15 secs later? or 43 seconds later?