In The Final Dossier, Mark Frost does not put Diane in the Great Northern Hotel's boiler room with Cooper and Cole (although it seems that Ben and Jerry (!) are there). Instead, Diane stays at the Sheriff's Station and by the time Cole returns, she has disappeared in front of plenty of eyewitnesses in Truman's office.
We don't see her disappear in Part 17, but based on the book, you could conclude that Cooper going into the room and back in time (for some reason) makes Diane disappear. However, I suspect her disappearance was added to avoid having to give away a lot of answers. When we see her, there's clearly no memory loss and she knows exactly what she's doing. Just imagine if Tammy Preston would've been able to debrief Diane!
Strange. Perhaps Diane disappears from the Sheriff's office once she 'crosses over' at 430 with Coop and becomes Linda? It affects the past?
I'm surprised there's this much detail about events connected to the final episodes. I expected it to be more about secondary characters like with Secret History.
The first mention, on p. 112, says that Diane "disappeared right along with" Cooper. But then, you're right, the second mention of events, on p. 131, makes it sound as though Diane disappeared after Cole and Cooper.
Perhaps Cole has not told Tammy about Diane being in the boiler room? But, he did tell her about the Horne brothers being there? Jerry, too? I thought he was naked in Wyoming...
So, the best bet would be to tie this into theorizing about timeline wonkiness more generally, I guess.
Something interesting to think about.
Fire walk with