I have a feeling she's Cabriana. Cabriana is dead,after all, but no one knows that except for Semirhage and Aran'gar. In LoC(I forget exactly where) it says that Semirhage and Mesaana always worked together. So couldn't Semirhage have tortured Cabraina to get all the info outof her and then Mesaana dressed up as her in the Tower? No one would notice, because so many sister's returned to the Tower after the split who hadn't been there before. Just a thought.