Yes, Diane's last level is already in the game, you have to pick the new option in the cabin!
The current hint is sleep in the cabin, which you do by draining or getting drained on a super drain day. I do so and get the option to take off the pajamas and then sleep. Upon waking up, I can read a note and proceed with the next day. These are the only options presented to me unless I'm blind (which I tend to be lol) and after doing everything I mentioned even multiple times, I'm still stuck with the same hint and at level 3. Same thing happens when you get drained instead. I'd also like to note that it's kinda strange you get the stretched pajama note when you get drained yourself despite not trying them on.
Ah, the book was in the section before the last so I thought I had read all the books since it's considered less complex than the most complex books. Oops... Also, I'd like to report that just resting in the cabin after going to it from the forest yields the "You won yesterday Lea, but I'll win other day!" (sic) note despite no draining taking place.
Also, how do I get the event for ending up somewhere else? I put on the dress (idk if that's even relevant, just wanna make sure) and tried the forest path 20 times and it hasn't triggered. Maybe I'm just unlucky?
Thanks for the report, I changed the code a bit so that If I release a bugfix that phrase doesn't happen if you haven't drained her that day!
For the forest thing you may be missing one more book, I have to admit that it's one of the most convoluted things of the game, you need to rent and read two of the complex section books, "Hartow's curse" and "The legend of the witch" once you have done that you can reach the new place.