Referring to CE 486....It's obvious that the west window would have allowed plenty of sunlight to enter the room and if the man was standing just a few feet back from the south window he would have been illuminated by the sun shining through the west window, and thus he would have been clearly visible to Arnold Rowland.
Robert Harris always has this thing where he prefaces a baseless assertion by leading off with the work "'obvious[ly]" in a vain attempt to avoid having to support is Shinola. Apparently, he's been rubbing off on you.
How is it obvious? The primary source of light in those photos is the photographers rig, whether it be due to speed lights or a flash bulb. Anyway, the sun at 12:15 is almost directly from due South. While the TSBD isn't oriented exactly N-S-E-W, it's fairly close, and what little sliver of direct light through the west windows would be cast towards the North side of the building.
The link below contains a photo taken later on the afternoon of November 22, 1963, from a perspective fairly close to what Rowland would have seen from his position. Since it's later on in the afternoon, the sun is shining from a much more Westerly direction than it would have a few minutes after noon. There should be a great deal more light coming through the West windows than at 12:15. So where is all that glorious, rifleman-bathing natural light seen through SW corner window? And where in his testimony, affidavits, and interviews does Rowland describe the influence of such light?
http://darkroom.baltimoresun.com/2013/11/john-f-kennedy-anniversary-a-look-at-images-from-his-lifetime/president-kennedy-gallery-9/#PhotoSwipe1613588479229
