There is no evidence that Graendal is more skilled with saidar than Lanfear, and Lanfear is at least two 'levels' stronger than her. Graendal only has a major Talent in Compulsion from what we know.

As for mental stability, how is Lanfear with her unstable temper more crazy than a woman who turns everything into a sex zombie?

I'm saying this here because I'm not up for challenging most of the other posts.