Well, I think her collapse might have happened after WH. Moridin has become worse and worse towards his Chosen as the books have progressed. In WH he was still civil to them, and they treated him with only borderline respect. By ToM he's treating them with outright hostility and punishing them as liberally as any other darkfriends. Within a few books Moridin took all the 'glamour' out of being one of the Chosen, and turned it into a daily chore with failure literally terrifying them and making them want to flee. If Moridin is so hostile to Graendal, I can't imagine what he's doing to two women whose souls he has around his neck.

That said, I don't think there can be much doubt that all the Chosen are as evil and corrupt as people come. As Graendal said, they weren't always totally corrupted like Semirhage was, but they were all primed for the Shadow even before He was released. They're all like Semirhage now, beyond salvation.

Salvation is not really what I'm alluding to anyway. Loyalty is hardly a virtue shown by any of the Forsaken. All of them are there for rewards, and Mierin in some ways more than others to be glorified. Demandred specifically left for the Shadow because the grass was greener on the Shadow's side. If Moridin removes all the rewards from the Shadow, I don't think it would take much for people like Lanfear and Demandred to defect. Not that there would be a point - they can be tracked as you said, and the Light would never have them. Cyndane may know this, and may try to use her previous association with Rand to 'save' her from both the retribution of the Nae'blis, and the justice of the Light.

But to be honest, I think it's all a giant trap... Cyndane is playing her old tricks in TAR again, and she hates Rand utterly. Still, I'm not putting the idea of defection by a Chosen out of my mind before the end of the series. Moridin has crushed them in his grip and even Graendal is losing her composure. It's not a new idea - Asmodean was a traitor under the right circumstances and the other Forsaken believed it without blinking. I'm convinced someone like Demandred will thwart the Shadow in some way before the end of the series through some delusion that another one of the Chosen is getting too close to Rand and robbing him of his 'glory', and Moghedien might end up being so terrified she'll end up being completely useless. I'm afraid the Nae'blis hasn't been very good up to now... most of the best Chosen are dead and the survivors are broken and a pale shadow of their former selves. I fully expect Graendal to be a traumatized head case in the next book, much like Mesaana was in KoD.