Hm. So, having read the story a little more thoroughly, I'm guessing that the princess is possessed by a demon. Otherwise I'm not seeing the point to Samarra's explanation of the Fomori and why the story went out of its way to tell the player about the backstory which, while interesting, is not really directly relevant so far.
I assume the crone is her sister and it's possible they switched at some point, though I'm not sure what the point of that would be. However, it was noted that they looked almost identical. Also, the crone hints very strongly that she and the princess know each other very well, but aren't on good terms anymore. However, this is only on the princess' side of things and the crone really wants to help her. And I suppose the woman at the river is probably the crone without the chameleon scale disguise. Maybe. Not convinced on it actually being a disguise instead of something more complicated. The crone's insistence on good manners is a bit at odds with her supposed character type, which is more often than not fairly foulmouthed themselves.
A body was found, but since this incident sounds familiar to Belle it seems likely that fey magic is somehow involved or some other kind of trickery. Assuming that the princess really is possessed, it would explain why the fairy queen has to wipe out humanity now or bang Dickon, apparently, since I assume that she'll end up sealing the demons away again like her predecessor before her.
What I don't really know is how Selena would have gotten possessed in the first place, but if the crone is her sister then they presumably both have magical talent that one or both may not have controlled very well.
