So, I'm revising my theory now.
I'm going to keep Sinclair=TTA and go back to Sinclair=Jessica as well. My reasoning for this is something Zander wrote over in the Entry 43 thread, which meshes well with a theory I had had previously and kind of scrapped at the time.
Basically, Jessica was a sort of replacement Masky for Tim, since he couldn't very well follow Jay around with a broken leg. This seems to also tie into the idea that TTA is also a group of people, or possibly that there is someone else pulling the Maskies' strings.
Now, for Sinclair/Jessica=TTA, I'd say she's only the current TTA. A couple have commented before that the recent TTA videos have a different style than they did in the first season. I think there was someone different (possibly Tim) uploading the TTA videos in season 1. After Jessica disappeared from the hotel is when TTA started posting videos again and if Jessica was acting as a Masky before, it could be that she ended up with them again and is now the one posting the videos. Though of course, I suppose that would imply that Jessica got her memories back (or at least part of her memories).
So, alternatevely, maybe Tim (or somebody else) is posting these videos and wants Jay to go through the footage quicker so that Jessica could get (part of) her memory back?
I suppose there are a number of possibilities here, but I think I stick with Jessica being our current TTA until I see some specific evidence otherwise.