[gundog]

And there was s roomful of LEOs overlooking that part of the pitched roof before the shooter climbed up there. Why did they leave?

[rx]

I believe the video (and people in that video) of the 2nd floor room were from an earlier day on which the USSS was doing its pre-event familiarization.

It seems plausible to me that on the day of the event, there wouldn't be need for nearly as many USSS personnel. We do see that in the space below where Crooks was positioned (notice I didn't say "positioned himself"), there were within the hour post-shooting, a number of LE/USSC personnel in that space.