It's a cool idea, Moog. That epigraph note makes me think that, if it fits, it would be one Herald planning all of this out, not an organized thing. If Jezrien murdered Taln and then convinced the rest of his plan, they'd be much more likely to accept it as he is their leader. Jezrien could be the one that is the traitor. I had assumed it would be Nalan for other reasons, but that works too.
As this is obviously a theory, I don't need to tell you how unlikely this is or question the stability of your evidence - if you had all of the proper evidence, there would be no discussion, because it would be fact - so I will just say that it's an interesting thought and definitely not impossible.