The scene with Diane/Cooper in Episode 18 has no relationship to the following scene of Cooper waking up, then going to find Laura. They are cut together to confuse. To understand, you have to parse them out and see that they are telling a completely different part of the story.
Cooper/Diane scene is actually Evil Cooper exiting the red room for the first time in 1991. He meets Diane and goes to a magic place he's led to by 430. He then has ritual sex with Diane.
This is the same sex ritual mentioned in TSHOTP, that Jack Parsons was trying to use to summon the mother. When Evil Cooper and Diane conduct this ritual, Mother enters the physical world for the first time and ultimately inhabits Sarah Palmer.
This is why Diane sees her tulpa at the hotel. The tulpa she sees is the same one we later meet, that ends up with Gordon Cole and Albert. Years later she tells a different story about her rape either because she is programmed to, or because she doesn't fully remember.
The next scene of Cooper waking up, actually immediately follows the first new scene of Season 3, in which the fireman gives Cooper clues and tells him it is "in our house now." This presumably means Mother or Evil Cooper has entered the White Lodge and is about to destroy it.
One clue to this is the scratching sound Cooper hears when he's trying to save Laura in the old timeline in Episode 17. That's probably the moment Cooper is supposed to the Fireman, who tells him the final clues, before he wakes up in Texas in Episode 18.
With the White Lodge on the verge of destruction, Cooper is sent to Texas to find the hidden Laura Palmer, which the Fireman hid there decades ago by creating the Laura orb. When he sees the note by the bed, referencing Richard and Linda, he immediately understands because the Fireman has just told him this clue.
Cooper finds Laura, and takes her back to the House, but Tremond/Chalfont has played one last trick. Cooper and Laura are stuck.

If true, this is a very dark ending and probably means that the Fireman and the White Lodge are destroyed by Mother. Alternately, the Laura dream theory could also be true. When Cooper takes her back, Laura ultimately sees through the Chalfont/Tremond deception and when her mother calls to her it has become a dream. In that possible ending, Cooper succeeds in his mission.