The questions about the why, when and how an MC rifle with useless scope was placed on the 6th floor, are I must admit, problematic to resolve as part of a CT theory.
As for the man with rifle at the SW window per Arnold Rowland, that’s at 12:15, then he is not seen again by Rowland. So this gunman moved to the SE window by not later than 12;24 so as to have time to put the box in the window before it’s recorded in the Bronson film approx 12:25.
So it’s probable that the black man that Rowland saw in the SE window at
12:15 was BRW who leaves that window by at least not later than about 12:23.
So a heory of a shooter with semi auto rifle at SW who is the same who moves to the SE window would be supported by Norman hearing only 3 shots from that SE window and not any other more distance noise by a second shooter at the SW window.
If the operation of the East elevator gate and slowing elevator adds 15 secs = 75 secs total time it’s still possible that TrulyBaker arriving at 70-75 sec to the elevators at ground floor would not perceive East elevator had been in operation a few seconds prior.
There might be a ? about if the cables were visible and If any lateral swaying occurs for a few seconds after and elevator cab has traveled 3 floors upwards.