I rewatched them, and Part 18 was even more powerful and haunting the second time around. A few things immediately stood out:
1. Watched outside the context of initial expectations, and knowing what Part 18 would actually be, Part 17 functions almost perfectly as a stand-alone finale for both the season and the series, replete with its circling back to FWWM and the pilot, and ending with Julee Cruise. It's essentially the "curtain call" Cooper referenced before going inside the room at the Great Northern.
2. Speaking of the Julee Cruise scene -- it functioned, basically, in the same way that all of this season's Bang Bang Bar/Roadhouse outros functioned, but was set in the Red Room. Does this lend credence to the theory that those Roadhouse scenes existed outside of the "normal" world of the show?
3. I'm still not sure how "real" Richard and Linda were. When Cooper wakes up in Odessa he still calls out to "Diane," and is confused when he reads that the note addressed to Richard. More (or less?) significantly, Sheryl Lee is billed separately as both Laura and Carrie in the final credits, while Dern is only credited as Diane. This discrepancy also points to the way Lynch used Kyle McLachlan's special billing in the credits to further blur the lines between the various Coopers, since none are ever identified by name.
4. Everyone is calling the final scene bleak and despairing, and it no doubt is. But what do we make of the significance of the scream causing the electricity to go out in the Palmer house? All season long, electricity has been associated with the Lodges, its buzzing nearly always indicating the presence of malevolent forces. The power going out would also, of course, cause that damn ceiling fan to stop spinning.
https://www.youtube.com/watch?v=63VHeu5V0zA
(note how this scene ends, also)