I never said that any of the theories indicated Graendal was the killer, but they could. My point was that they are more convincing than the one you have presented.

By the way, the powers the Shadowspawn have may be unrelated to the True Power or the One Power - Aginor did something in their creation that gave them these abilities; we don't know what or even if they are a result of a combination of those forces or an effect of those forces on certain genetic structures. Just as the Dark One does not seem to have used the True Power to do any of His more powerful version of reality twisting. We basically don't know how they have these abilities. Just because an ability is derived from the OP, TP or the Dark One or connected to them doesn't mean it is created by them. A good example is Dreaming, which does not require channeling, or the taint at Shadar Logoth, which is an ancient force of it's own.

And, IMHO, the Dark One already has far more powerful and silent assassins in Moghedien and Lanfear. It's part of their modus operandi to kill targets using T'A'R and the One Power. More useful than some guy with personality issues and a knife.

You might need to re-read the Battle of Two Rivers. Unlike Rand and Mat, Perrin has dealt with Slayer directly, and Slayer was directly assaulting the wolves. There is more tying him to Slayer than the other characters.