If we assassinate her and she's not the Stalker, then we know the mayor certainly is.

Another thing to take into account is that the Enthrallers take the passions, interests, jobs etc. of the Pokemon and turn them into something twisted. Was Millie into reading, or poetry? And does that have anything to do with hearts? The create/destroy thing with the poems fits with Millie possibly fighting off the Enthraller. She didn't deny writing the Twisted Note, and she did act strangely when the sun was setting. Repeating her own name could be seen as Mille trying to fight off the Enthraller, or trying to reaffirm her identity if Shadefetch somehow got to her. Also the Iron Key in "Koax's" room led us to that room with the Disorder Door. That hallucinogen seemed Shadefetch related. There's something up with that...

And on the door was an inscription: NO, YOU'RE USELESS!!!!!