I will add to two cents to the Mesaana identity quest.

We know the Mesaana is still in the Tower. We know Mesaana is posing as an Aes Sedai. This has been confirmed by Brandon at a book signing and from Graendal's POV.

After the Forsaked broke free they went about establishing power bases. For the most part they exhibited the same modus operandi.

1. The Forsaken became the powers behind the throne. A new lord would appear and very quickly become the advisor to the king/queen. Through manipulation, and compulsion the Forsaken would rule.

Be'lal became High Lord Saman and the power in Tear.
Rahvin beame Lord Gaebril the power behind Morgase in Andor.
Sammael showed up as Lord Brand and quickly became one of the Council of nine in Illian.
Graendal sets herself up in Arad Doman and uses Compulsion and forged letters to disrupt the country.

The powerful positions behind the Amrylin Seat are The Keeper, Mistress of Novices, Sitters, and Ajah Heads. Mesaana would be one of these.

2. The Forsake love their creature comforts. Be'lal, Rahvin, Sammael, Graendal all set themselves up with castles, servants, fine food, fine wine, members of the opposite sex, and whatever other comforts they would want. Many Forsaken POV's lamented how the comforts of the Age of Legends was so much better than this Age.

Who in the White Tower has exhibited excessive comforts?

3. Aes Sedai have long memories. When the SAS wanted to verify Siuan and Leane's identity the Salidar Aes Sedai asked Siuan and Leane questions of conversations made 20 years ago. Cabriana Mercandes was tortured to get information so Halima could pose as Delana's secretary. Anaiya and Kairen were murdered because they were friends with Cabriana and might notice inconsistencies in Halima's story.

Out in the mundane world the Forsaken could use compulsion resolve the issue of a lord no one heard of quickly becoming a powerful lord. In the mundane world compulsion could be used to hide inconsistencies of the people you know well. Compulsion in the White Tower is another story. Aes Sedai and other users of the One Power eventually break free of minor compulsion. Major compulsion would leave an Aes Sedai nearly brain dead and every Aes Sedai would notice that. Delving might detect the use of compulsion.

Mesaana could not just show up as some unknown Aes Sedai and within a few months be a Sitter or Ajah Head. Mesaana would have to show up as a known Aes Sedai who has been gone from the Tower for a long time. That way minor inconsistencies with behavior would not be as noticeable, or if noticed could be excused as people change.

Who in the Tower has been gone for a long time, did not have many close friends, recently returned, is in a position of power, and really enjoys an excessive extravagant lifestyle?

I give you Tsutama Rath the Red Ajah Head. Tsutama was exiled to a remote farm after the "vileness" following the Aiel War and just recently returned. Tsutama was raised to be the Red Ajah Head after Galina was believed to be dead after Dumai Wells. Tsutama has flambouyant quarters. From her three clocks, tea cups made of pounded gold, to tea with a hint of out of season blueberries, Tsutama's quarters are extravaganat. Pevara notices small changes like the coarsening of Tsutama's language.

Tsutama orderd the Red Ajah to send sisters to Taim/Black Tower to bond Asha'man as warders. If the Asha'man bond the Aes Sedai the forces of Dark get stronger and the forces of light get weaker. Just one more way for the White Tower to get weaker.

Tsutama was not one of the Ajah Heads to decide Egwene would be the Amrylin Seat. The Red Sitters were not part of the Hall to elect Egwene. Tsutama was not able to delay or divide the Tower further by preventing Egwene be raised to the Amrylin Seat.

Of course in RJ's balanced Randland Tsutama being Mesaana would be to much dumping on the Reds. The Blues have Moiraine and Siuan as hero's, the Greens Elayne and almost Egwene has heros, the Browns have Verin (I know she is Black but I will consider her as Brown)as a hero, the Yellow have Nyneave as a hero. The Reds have Galina, Katerine, and Elaida.