In the prologue to The Gathering Storm, Graendal commented that Mesaana, "was in the White Tower, pretending to be one of what passed for an Aes Sedai in this Age," and that she had agents there keeping her carefully apprised of what Mesaana was up to.
So, she is definitely posing as a full sister (and as per Sanderson's comments, is still in the Tower).

To me, unless Graendal has been carefully watching the wrong person all this time (though getting the venue right - unlikely), this absolutely confirms that Mesaana is posing as a full Aes Sedai, and one that was (apparently) loyal to Elaida's faction - not an Accepted, novice, or servant.

No?

(Also, regarding the Mesaana's plain white dress means she's a novice argument, even disregarding the above, there is no real evidence that this is her normal garb; however, when Alviarin used the red rod to summon her in CoT she appeared almost on the moment - and was in silk).

Not really going to try to go far beyond that here in working out who exactly she is posing as - I've always subscribed to the traditional Danelle-or-just-possibly-Norine view and really continue to do so, but this at least conclusively rules out Teramina and the other two in the Epilogue, as well as Evanellein.