Regarding Sarah/Judy. This, in my opinion, is the key to unlocking much of the mystery here.
The noise that the frogmoth makes in Ep 8 is the same noise that occurs the moment Laura vanishes from the FWWM timeline, and this occurs immediately after Sarah destroys the photo of Laura.
Mr. C makes it clear he is looking for what we assume to be the frogmoth symbol (black circle with tiny dog ears). The coordinates he searches for lead him to the white lodge with a destination on the screen of Sarah Palmer's house, that is then changed by the Fireman to the Sherriff's station.
These things, in addition to Jeffries telling Coop straight up "this is where you'll find Judy" before he goes back in time, and the girl in 1954 being the right age range to be Sarah, AND the shit we see when Sarah takes her face off- she's got to be Judy.
I feel like the answer, or more insight into what actually happens in those final moments of the finale is firmly rooted in the Lodge scenes/Audrey's scenes in other episodes. Because even as far back as the S2 finale, people in the lodge (Laura, specifically) make references to the fact that one moment they are not who they appear to be, but the next moment they are. Carrie is not Laura but she is. In what way, through a dream, a timeshift, or a reality divergence, we don't quite know, but the things she says about (paraphrasing here) being a long way from there (Twin Peaks?) and being too young to know what she was doing seem to directly reference being Laura. But then she doesn't recognize the town, or the house.
Maybe the Final Dossier will map the road, but I feel like we have the information scattered about in the episodes now.