There is no reason for the shooter to approach the SW window with a rifle at the ready position unless he was intending in take up a position at that window to prepare to shoot from that window.
Just because Rowland doesn’t see a gunman at the SW window again after about 30 secs does not mean necessarily the gunman left the window and never returned. He might have just moved out of LOS by hiding in the corner just right of the SW window which placed him in total shade.
If this is the scenario, then the bald elderly negro at the SE window was probably not BRW , but more likely a 2nd gunman, and was in process of preparing that window by placing a box and then lowering the window to the 15.5 inch height. He was 1st glimpsed at 12:15 and then at 12:20 by Rowland. At 12:25 when Rowland looked again, there’s no black man anymore.
Could this indicate that at 12:25 the box has been placed , the window lowered , and the black man is hiding or sitting on the floor and out of LOS?
If he is a 2nd gunman, however , ( to shoot the MC rifle to frame Oswald?) then surely Brennan and Euins would have IDd the shooter as a “dark completed” male with a noticeable bald head?
Only Euins comes close by describing a bald spot, and there’s some controversy as to whether or not Euins ever made an initial statement that the man as a black man.