I don't think the issue is that Mesaana is Forsaken, so must as the fact she's brain damaged beyond repair. There's not much point, or honor or justice in executing a drooling, idiot vegetable. We know, from various references in the series, that a Forsaken in WT care would be excecuted, even a DF would.

Egwene was not in a position to get much of anything out of the captured BA. She's also got a string of bad experiences with trying to hold DF/Forsaken\BA together. Egwene got her lesson with Moghedien, with the BA in Tear --- and knows better now. The potential benefits, which are slim to begin with as what BA say can't be trusted, aren't worth the risks.