Speaking as someone who has been reading the books for years, Graendal never even crossed my mind as a possibility except when people at Wotmania suggested it.

It just doesn't make much sense to me. Sure, she had the means, and there aren't any facts contradicting it. But that would be such a low-impact revelation, especially when there are choices like Slayer, Lanfear, Moridin, Demandred, Padan Fain, and Shaidar Haran. Every single one of which has more opportunity for dramatic impact. Graendal seems like a boring, easy way out to me, and I can't think of a situation where revealing that it was her is in any way important.

I think the idea that Graendal is an obvious choice only works if you're coming from the point of view of a really big, analytic fan that actually spends hours and hours theorizing outside of the books. I've never gotten the impression that Sanderson fit that description.