I believe Laras is Mesaana. RJ talks of Laras in one of the book glossaries (sorry-can't remember which one, but it is not books 5, 6 or 7) and says there is more to her than meets the eye. She is a servant, but with authority (not that any AS would deign to really notice her), and she can disguise herself much as Lanfear did in Rhuidean. She helps Siuan escape to decrease Elaida's control of the situation, while increasing Alviarin's influence. It would also potentially contribute to a divided White Tower, with Siuan loose. Something to which any forsaken would like to contribute. Even if Siuan died shortly after escape, there is nothing lost in the DO's if the stilled former Amyrlin is released, but there is much to gain if things work out well.