I've never put too much significance on the fact that photos of the TSBD taken from Houston Street do not show anyone in the sniper's nest. If you've visited the 6th Floor Museum you can see that there was plenty of room between the widow and the wall (even with some water pipes in place) for a man the size of LHO (for example) to sit and be totally hidden from view. If indeed the shooter all along intended to wait until the limo had turned left on Elm there would be no reason to expose himself while the limo was on Houston. If the shooter wanted to see where the limo was on Houston, he could have stood back from the window in the shadows and gotten a great view.
All in all, I don't think the absence of a person in the sniper's nest window while the limo was on Houston proves anything one way or the other.
Proves anything? That would depend on where you feel the position of the limo is. Meaning, for example, when the Hughes film stops where exactly is the position of the limo while at the same time you see the 6th-floor window empty. When the Hughes film stops the limo is on ELM in front of the TSBD main entrance. Seconds later the first shot, but how many seconds? Because of course, he knows the Hughes film stops and only has a few seconds to come out of the shadows to position himself in the window and fire off the shots. The shooter wouldn't have time to just shoot at the floor. If it was Annie Oakley, then maybe