Mesaana has been in the WT for quite some time - Edit 1
Before modification by Light in the Night at 17/11/2009 06:37:35 AM
I don't recall (I am old and forget things) a mention of Mesaana being in the Tower before Lord of Chaos. I thought Mesaana used the Black Ajah to get rid of Siuan and install Elaida. Then used Alviarin and Galina to get Tsutama recalled. The timeline works I think.
aCoS was when we first see her, but Alviarin mentions many meetings with her before.
Clearly, she has been there. If her presence in the Tower is unnecessary for her to control the BA while deposing an Amyrlin, she doesn't have to be there at all. I think it is clear that she has been in the Tower since the events of tSR.
In LoC chapter 6, Graendal wonders how long Mesaana has been in the WT. (This was after Mesaana had let her hiding place slip out in a meeting with Graendal and Demandred.
And, in LoC prologue, we have Demandred saying this:
Still Demandred kept silent, stood there studying them. No, not Graendal. Semirhage and her. And when he did speak, half to himself, it was to they two. "When I think where you two have placed yourselves, I wonder.
The obvious inference here is that Demandred knows that Semirhage is with the Seanchan and Mesaana is in the WT.
For fairfax:
Tsutama Rath was in exile, along with Toveine and Lirene for the last 15 years. (For being involved in the "vileness" after the Aiel War.) The first we hear of Elaida's plans to send a party of sisters to the Black Tower headed by Toveine is in ACoS prologue. However, we don't know exactly when Elaida could have sent to retrieve the 3 ex-Sitters. She may have felt guilt ( ? ) that they got punished while she walked free, and sent for them soon after becoming Amyrlin.