I never really had much of an opinion on it, until just now when reading Leigh Butler's WoT reread, because reading a synopsis like that helps with seeing the bigger pictures in the plot and not just what happens from chapter to chapter. And the thing is, Rahvin had that plan with Graendal and Lanfear (and Sammael, sort of). When Rand came for him, they were to be there. They're the only two candidates who we know had a good reason to be there. And then as for choosing between the two, well, obviously Lanfear would make the most sense with regard to dramatic tension and the "You?! No!", but considering what happened earlier that day, Graendal seems more logical, as any theory involving Lanfear requires massive assumptions that have little support in the text to explain her getting back so fast; it's not impossible, but RJ's claims that the clues are all there would be a little strange then. If Cyndane ever gives us some more substantive insight into what happened to her, that should settle the debate once and for all either way, even if Asmodean is never mentioned.