Is it possible Mesaana was one of the sitters in Egwene's camp? One of the young ones?

It could be possible, especially since Balthamal/Aran'gar wasted so much of her time seducing men that she may have ignored signs of another Chosen being there. Remember that s/he completely overlooked Moghedien being among them until told that she was there and being commanded to rescue her.

I mean, how DID so many of the black ajah in Egwene's camp find out about the plan to capture them unless one of Egwene's sitters is Mesaana?

My impression from the morning after was that it was a particularly bloody day and almost every Black Ajah Sister was caught by surprise, Shielded and executed. It seemed to me that it was the BA in the WT that received a warning allowing most of them to arrange an escape.