Do you think Mazrim Taim was turned by 13 Myrdraal? If not, was he a darkfriend before he was rescued by the black ajah? Supposing Mazrim Taim was not a darkfriend before his rescue, the rescuers could have given him a choice of either death or become a darkfriend...kinda like Verin.

But there is one important difference between MT and Verin. Verin could be forced to keep her oaths to the DO with the oath rod. Without access to the oath rod, any oath given by Mazrim Taim would not be binding. Thus, I believe that to be safe that MT was on their side, the Black Ajah and/or Forsaken turned MT with 13 Myrdraal.

I'm fairly certain this is what happened. The question is, did Verin know of it and if she did, then Egwene almost certainly knows of it too via Verin's book.

Now that Egwene has the unanimous support of both factions of Aes Sedai, can Egwene safely reveal what she knows (if she knows) about Mazrim Taim?

My guess is that Egwene will leave it up to Rand to deal with. She'll be trying to organize for Tarmon Gai'don.