Eris is part elf (dark elf, specifically, as shown by her eyes, and yes, that's why she didn't get the job at the cafe), but we have no reason to believe as yet that she's a half-elf. Lots of people have some dark elf DNA (including Ashley and Ellen) and have no problems at all (at least, not related to elvish background). Quarter-elf and below are (currently) safe; half-elves are less than dogs, elves are slaves and have some (though few) rights.I think part of the issue is, isn't she part elf?
No real opinion whether she'll fill the last room, but I wouldn't object. I don't see her in the sanctuary unless more comes out about her.