Maybe Mesaana is not masquerading as an aes sedai, but instead is a servant or similar low key figure.

I find it hard to believe that Mesaana is masquerading as an aes sedai, because dozens would know her, from years in the tower in training, tests for accepted and aes sedai, for example. And it's tough to weave an illusion that doesn't look closely like oneself, so it would be really hard to assume someone else's identity.

Lastly, when Alviarin sees Mesaana revealed, she can almost recognize her, but can't make the connection. Who more likely to be "almost" recognizable than a servant?