Seems most of the eye-shielding people are looking towards the south. Nearly all those in the Altgens photo looking to the southwest, as the Sniper's Nest gunman would have been, are facing away from the sun and not shielding their eyes.
So when you wrote "How could a gunman make any kind of an accurate shot facing into such glare?" you either didn't think about the angle of the sun and it's relationship to the limousine position on Elm, or you think a gunman was aiming at the motorcade down Houston Street or towards the reflecting pool. A teachable moment if you want to grasp it.
Teachable? Meaning- just look at a map and never mind a photograph? Facing away from the sun? The person in the Altgens photo-- on the front steps apparently shielding his eyes with
both hands [between Lovelady and the SS agents on the right side of their car].. is not looking at the motorcade?
The shadow from the running man illustrates the direction of the sun ...directly towards the suspected window--