No, Tuon never wears a sword and why would she help Aes Sedai?

I believe the Seanchan woman in question is Eaganin, who used to wear a sword across her back before she moved on to other things, and likely go back to it now that she abandoned her Seanchan ambitions. No other Seanchan woman really fits.
I am currently rereading the series and was struck by the fact that in tSR Nynaeve quite inexplicably "forgot" to tell Egwene about the Seanchan woman who was helping them, and notably how in tGH Egwene is the only one of the four women who does not get to see Bayle Domon. Seems like a setup...

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