I haven't followed the Mesaana identity debate too closely, so pardon me if this is old news.

However, I have picked up that there is a rather fierce discussion around whether Mesaana is posing as an Aes Sedai, or as some other woman in the Tower.

Well, in the Prologue to tGS, Graendal spells it out for us.

While moping over the fact that she has no clue about what Demandred is up to, Graendal says that Mesaana is posing as one of the so-called Aes Sedai of this age, and that Graendal's agents in the Tower are keeping her up to date on Mesaana's actions.

So it seems the Aes Sedai/not Aes Sedai debate with regards to Mesaana is actually no debate. Graendal has settled it once and for all.

Mesaana is posing as an Aes Sedai. So the only question that remains to be answered is which Aes Sedai?

Glad I could help. But I'm off after more interesting stuff. Like finding a way to prove that Semirhage is more powerful than Nynaeve.