I've said it before, but give Marceline time. She seems to hold the most benevolent interest in Finn, and she's not getting any older. Finn OTOH is getting older, which means he's getting closer to Marceline's physical age.
And yes, PB seems to be getting progressively more unstable. I've seen things where people theorized that she was a mutated Betty that went just as nuts as Simon/Ice King but in a different way. Granted, the fact that she has talked about her candy ancestors makes that unlikely, but there's nothing saying she might be off in her own way. Between her being possessed by the Lich and the episode where she had Finn and Jake lock up Ice King for no reason, I've always had some reservations about her.