I think it might be the ghost of Marisa's mother or just some ghost using those memories to torment her in her sleep.
In reference to Liar's Paradox, though it's actually newer than this one, I think that's correct. Not sure if "not the real thing" means that it's some other spirit looking like her, as you say, or just that it's only her spirit and not Youkai!Mom come back to life.