That the majority of readers would agree with you that it's plausible that Graendal was there. But IMO it isn't, especially because we hardly knew anything about Graendal before LoC, except that she is a very obscene Forsaken. For example, Sammael would have been a more obvious candidate before LoC.

Also RJ's comment that the two servants Rand sees when he hunts Rahvin are in no way connected to the murder makes it rather unlikely to me that nonetheless Graendal was there to Compel any servants, unseen to the readers and that this is the reason for the murder.

Besides, if Graendal searched someone to use Complusion on to gather infos, it only makes sense to strike with heavy Compulsion on Asmo, abduct & interrogate and then kill him. This would be a perfect opportunity to Graendal to learn more about Rand & his plans. Like she tried in PoD with Cyndane & Moghedien.

As said, if balefire was used, it only makes sense that Graendal would use very weak balefire, but we know that in this case the DO can transmigrate the balefired person (but concerning Asmo, the DO couldn't).

It wasn't a big mystery, but I still think that RJ had a precise idea of what happened to Asmo. That a killer, who uses TAR, takes him there to die and get rid off his body, isn't such a complex thought. And that RJ indicated that he will most likely reveal it in the PoV of the killer speaks against Graendal as well because she is most likely dead and won't have another PoV.

It also makes sense to consider that the same killer, who already assassinated people associated to the Shadow twice before in the series, namely in the Stone of Tear and in Tar Valon, struck again in Caemlyn.