Carrie Page isn't Laura; she is a younger version of Sarah Palmer

Hi everyone, I'm the guy who wrote

https://www.reddit.com/r/twinpeaks/comments/8q3jsd/all_everything_becomes_clear_on_a_rewatch_part_1/

...and who may or may not be the Fireman.

As I explained 7 years ago, Episode 18 of The Return doesn't take place after Episode 17; rather, it's a replay of Season 3 in a timeline where Laura disappeared.

Cooper is looking for Laura; he follows the clues given to him by the Fireman and arrives in a parallel world, where he meets a woman who bears a striking resemblance to his target. Naturally, he assumes she's Laura--but in fact he's in the right place at the wrong time, having found Laura's mother instead. She doesn't remember her daughter because Laura hasn't been born yet. In an attempt to jog Carrie/Sarah's memory, Cooper drives her to Twin Peaks, still believing he is in the same year that he switched worlds. When he finds out that the Palmer house belongs to Mrs. Tremond, his first thought is that the Palmers have moved out--but they haven't bought the place yet.

Slowly, he begins to realize he's arrived in the past (“What year is this?”). Meanwhile, Carrie/Sarah hears Judy's voice calling Laura. Judy is somehow connected to the Palmer house and can travel there using electricity. The scream is Sarah sensing Judy's arrival; the screen going black indicates that she's been possessed.

So basically, it's Cooper who sets the cycle of violence in motion again. If he hadn't searched for Laura, Sarah/Carrie would've never arrived at the Palmer house and would've never been possessed by Judy.