While the Drowzee family gets Forewarn, that's not why it's there. Forewarn's Japanese name is "Prophetic Dream". I don't think I need to go on from there. Though it is odd that's there though. Like aside from that and Cresselia, all these research books are about nightmares and negative things concerning dreams. Like maybe instead of Forewarn they could have included the move "Nightmare".
But
Codraroll and
stage7_4 are right, Darkrai is not specifically blamed. Though with that said, it was never said Darkrai was to blame for Eldritch's son. The son only says "Dar... Dark.. Is watching me..." and never says anything about the incident again. Who's to say "Dark" is specifically referring to Darkrai? Maybe he meant if was a "dark presence" was watching him, or maybe he was just saying it was dark and the "is watching me" part is unrelated (unless the Japanese version was more specific).
This is a semantic argument; it's Darkrai. Darkrai put the girl (and Eldritch's son) into an endless nightmare (a thing only it can do, Hypno would have kidnapped her also). Yes, the Lunar Wing is a general nightmare curer, but as its from the counterpart of Darkrai its specifically able to awake someone from Darkrai's eternal nightmares.
Maybe the reason there were books on Drowzee and Hypno is because originally the girl's parents tried waking her up by having them use Dream Eater to eat her dreams (heck, maybe THAT'S why she was slowly wasting away, her parents made a rash decision). When that didn't work they did more research and learned of the Forewarn Ability which the Drowzee family luckily had. Using that they figured out what was going on, researched Darkrai, that led to Cresselia, and that led to the Lunar Wing (note no where its mentioned about the Lunar Wing yet they seemed to know that's what they needed to get). As for why she didn't wake up even though the Lunar Wing at least got to the house, as I said it could have been too late. The parents got the Lunar Wing, rushed to her bedside, but by the time they got there she was gone. Finally I think she did play on the Marvelous Bridge because the old woman you speak to mentions her Abra. Her ghost does not have Abra with her, so how would they known that unless, when was alive, she did play with her Abra on the bridge.
Look, I can already tell we're not going to reach a conclusions as it's logistical proof vs. rational deduction. So how about we move to another but related Mystery:
This is the Lunar Wing:
This is Cresselia (and its Shiny):
So... where is their lime green coloring on Cresselia?