I think Lanfear starts out evil in the D&D sense (i.e. being motivated entirely by self-interest), not in the sense of being truly dedicated to the Dark One. I just finished re-reading her first couple appearances in tGH, and everything she does, especially in Cairhien, reads to me as though...
Certainly those attributes themselves wouldn't be working for the Light, but I can imagine someone inclined that way deciding to, nominally at least, work for the Light if he or she thought it was in his or her best interest to do so. In fact, the only two Forsaken who are in it for selfish...
The thing is, as far as we are aware, all of the premises on which Ishamael based his conclusion are true. Unless we've all been really misled about how the struggle between the Light and Dark work, then there is no sound argument to refute his position. That makes it vanishingly unlikely that...
See, I think that's exactly what makes it really unlikely that Ishamael would turn. When someone chooses a side for selfish reasons, there's always a chance that selfish reasons could cause them to change sides again, but Ishamael's decision is based on solid reasoning. Unless it is absolutely...