It's hard to tell from the videos and recordings I've seen. But if there is a change in the rate of fire, it could be simply due to his changing weapons. Perhaps a rifle jammed, and rather than clear the jam, he simply picked up another rifle. (That's probably why he had that many rifles up there in the first place, so he wouldn't have to clear jams.)

Sounds will echo in an urban environment. Even trained soldiers will have a hard time determining the direction shots are coming from just from the sound. Paddock was located because people in the hotel could tell where the shooter was. If there was a second shooter somewhere, why didn't they find him as well?

Granted, I don't put much stock in conspiracy theories. I'm just a boring person.

Onward and upward,
airforce