I don't think it is possible that Messanna is Verin or Delanna because they are both browns. My whole basis for this is in the prologue to LoC it says Messana is wearing a silk gown. We all know that browns seldom wear silk, there have been many refrences to this fact. We also know that its pointless to wear a mask of mirrors for clothes,so we have to assume that in the Forsakens meeting, they were all dressed as they would be otherwise. So, if Messana is wearing silk, we have to assume that she is not brown. This also means that she is not a servant because servants wear silk even less than browns. I vote for Cabriana, I don't remember what ajah she was, but I don't think it was brown. Messana would have to kill a sister to take her place without the threat of being discovered, or have Semirhage do it for her. If a sister showed up that no one had ever heard of and no one remembers from novice or accepted training, people would get curious and/or suspicous. Even if you don't have friends, someone has to know who you are. So Cabriana is a good choice. We just have to have some confirmation that Cabriana has been seen in the tower lately.