I'm currently leaning towards your third point (but am quite fascinated by your fourth point as well) as it would explain some if not all of
the observations I made… wow, nearly 50 pages back.
@LocoGringo78, I also read your response of course, which thoroughly lays out how the different girls came to playing a part in the dream sequences, but there's still Stacy's general demeanor. Being somewhat defensive, secretive even, her whereabouts at times unknown. And her slip of the tongue is really bothering me. And I think that her
also being the MC's sisters is a neat explanation (of sorts).
Though in the end, I may be reading too much into it and she's just going about her own perfectly normal business.