"Silviana = Mesaana" is a bold claim, but there are a few reasons I suspect her:

1) She gets to mete out punishment, something the FS love to do.

2) It's a powerful position, and she can blame Elaida for anything she does.

3) See seems above and beyond the Ajah disputes - this makes perfect sense for someone who sees the AS as children anyhow.

4) It gave her a chance to try to break Egwene. When Egwene didn't break, she decided to do what she claimed in the Prologue of tGS: "I will deliver ... an entire brood of channelers who - one way or another - will serve our cause in the Last Battle" and "More and more join our cause. Some intentionally, others unwittingly".

From the above quotes, it seems that she realises that a strong tower could be better for the Shadow if it can be turned to fight for the wrong cause. Who better to misdirect the tower than someone the Amyrlin trusts and relies on - the Keeper?

Mesaana can tell that Elaida will fall - so she goes out to the Hall and proclaims support for Egwene, knowing that Egwene will hear of this and reward her.

5) Perhaps one of the most compelling pieces of evidence - Silviana was the first Tower AS to swear on the Oath Rod. This means she could decide exactly what to say, rather than having to follow a set formula. In this way, she could make the oath that she wasn't BA and noone would question it - if the first oathtaker had said something like "I have never served the Dark One", then Mesaana would need to make a similar oath or risk sounding like she was circumventing something. By swearing first, she avoids this.

6) Finally, the part of the text that gave me the idea it might be her: In the Epilogue, Egwene is considering who Mesaana might be, and coming up with blanks. The very next sentence involves Silviana entering the study to take Egwene to see something - I might be wrong about the other stuff, but I know a literary technique when I see it.

I agree that Tsutama might well be Mesaana, but I think there is still some evidence for it being Silviana. Another possibility is that she was masquerading as a Black sister, and has left the tower - Duhara strikes me as a possibility, since she is now close to Elayne, and could free a cohort of imprisoned Black Ajah and then strike at Andor.