Colin
If Jarman and Norman did not go up to the 5th floor until 1220-1225, and Bonnie Ray Williams did not descend from the 6th floor to join them until after that, that can only mean BRW was present on the 6th floor, almost right on top of the Sniper's Nest, slightly before the motorcade passed at 1230.
How would Oswald have been able to re-arrange all of the boxes in the SN without BRW hearing or seeing him, as BRW was within spitting distance of the SN?