This was on reddit. Sounds like the most plausible solution off the show:
"Judy steals away Laura when Coop is rescuing her, and Diane and Cooper give chase. The real Diane knows Cooper-- maybe even loves him--and trusts his intuition, so she plunges in with him. They drive 430 miles and enter into Judy's pocket dimension. Here, they seem to be indistinguishable from their Doppelgangers--Diane retains the trauma of her Tulpa (which she sees from the car) and Cooper maintains the steely, frightening drive of Mr. C (further evidenced when Cooper fries the guns. He seems to be threatening the waitress with his posture, and also warns the innocent chef that frying them might end up firing off a bullet and injuring/killing him).
Unable to deal with this (or maybe even stolen away by Judy) Diane "slips" away into Judy's ugly reality and become Linda to Cooper's Richard. Cooper might give up here, as he's once again lost someone he loves in the pursuit of good. But he remembers the giant telling him to remember "Richard and Linda."
The slipperiness of Judy's lair, I think, is mostly meant to disorient or discourage Cooper (or Jeffries, or anyone else who dove in to find her). Hence the motel shifting before and after Diane and Cooper stay. Or Laura being off at work that day. Or her being a murderer in this universe. Or guys pulling guns on him. But Coop reads the clues, beats all of Judy's trials, finds Laura, brings her to the Palmer home where Judy is holed up. The final hurdle for Cooper--beaten, lonely, and exhausted from the very long drive (if he slept, Judy would snatch him too, presumably)--is that Sarah isn't there.
Cooper is almost ready to cave at this point. He's clearly shaken by the defeat. For the first time in this universe, probably in the entire show, he doubts himself, that he's "Special Agent Dale Cooper." Maybe he is Richard after all. Maybe he should just go home and sleep and then slip, like Diane, into his new life.
But instead he remembers the constant question: "Is it future, or is it past?" He, with his impeccable intuition, asks the question, and overcomes Judy again. Maybe it's an answer to a riddle, or maybe it's proof of Cooper's pure and unwavering nature. I tend to believe it's more of the latter, but either way, he doesn't give up and proves he's the hero through and through. This either directly allows or compounds with some sliver of Sarah reaching out to Laura, and Laura suddenly remembers everything. She becomes Laura again. And being Laura, with all she's gone trough, is so horrifying that it causes her to scream in agony. But Judy, regardless, is finally defeated because of her presence.
The endless fields of throbbing power lines we've seen Mr. C following all season go dead, and the electricity finally goes out in the Palmer house.
Which means that the fan stops spinning and hopefully will never start again. "