True, we only know for sure that the Forsaken and a few other non-channelers can summon Slayer. However, the fact that Slayer believes that it has to be a Forsaken kind of screams to me that it has to be someone else.

How is killing two sleeping people even remotely the same as how Asmo was killed? Slayer's main advantage in being an assassin is being able to get to anywhere, locked jail cells and inn rooms, not random hallways. Both instances the targets were not moving and he was given the info for where they were. To the best of my recollection we've never seen Ishy/Moridin kill anyone either, I guess he's not a threat. If that's really the best argument you have against Graendal, you might want to work on some other things. I'll agree with you that Luc, assuming the merging didn't destroy too many memories, would have a much better understanding of the Palace's layout. However, I would never argue that Graendal went looking for Asmo, I believe it was a chance killing. She was there either working with Rahvin before his death, or she came to loot the Palace after he died as she did Sammael's lair after his death and Asmo accidentally ran into her. She balefires him because she knows it's the only thing that can't be stopped, she reverses the weave which explains why Avi doesn't sense it, and Rand and Rahvin had been using balefire throughout the Palace explaining anyone sensing the residue later.

In theory, however, Asmo could have at least tried to dodge, tried to channel. He doesn't bother, just as Be'lal can't do anything when Moiraine balefires him. In fact, as I mentioned in another post their final death scenes are quite similar in description.