Theory about ghost *Spoilers*
I think what the ending indicates is that the Slit-Mouthed Woman isn't so much a singular ghost as it is a reoccuring demon. Think about it: in the scene where the Slit-Mouthed Woman begs her son to kill her, she says something to the effect of: "Kill me or it'll kill you!" It was a bit silly how she'd beat the snot out of her kids, and was then suddenly like, "Oh my God! I'm such a terrible mother!" It seemed to me that perhaps she was possessed by a demon. It would kind of make sense, especially since one of the mothers possessed by the Slit-Mouthed Woman was actually very kind to her children.
Elphaba lives!