Occam's razor suggested Graendal was responsible. Aside from the Slayer theory, all of the other possibilities required big leap of logic, or a huge piece of missing worldbuilding (like anything involving the Finns).
I considered Slayer on behalf of Graendal/Moridin as a decent possibility because it didn't require any of that. It was less likely due to needing 'opportunity' (assassins don't just hope to be in the right place at the right time, they have to make sure they are), but the more RJ fed this 'mystery' and refused to outright answer that the leading candidate was the correct answer, the more I kept thinking... maybe it wasn't her.

And again, if it was so irrelevant to everything else, why urge on the speculation for so long... and in the end provide such an unsatisfying resolution in terms of detail? Yeah, the readers became fascinated with the mystery, but you can't say that RJ didn't throw fuel on it for 7 books.