I like this idea, because to me, it creates more mystery and leaves you thinking even more: what else could be going on? The scale of this whole thing is enormous and I wouldn't be surprised (or rather, I would be) to see all kinds of twists and turns Brandon could throw into it.
Yeah, but twists should be relevant to the plot or they lose impact. We know that Rayse went to Sel and killed both Aona and Skaze from the epigraphs. We also know that Odium went to Sel prior to the events on Elantris. The reasonable assumption to draw from this is that Rayse is Odium. A twist could be thrown in, but it doesn't seem like Rayse not being Odium would enhance the plot.
Not everything should have to be spelled out to be considered obvious. Some caution is needed, but not everything is intended to be a big mystery. The author of the Epigraphs was clearly writing the letter to warn about the dangers posed by Rayse and asking the other power to intervene against him. Odium is (in parallel) set up as the main villain (or at least the current threat) and has behavior and history that agree perfectly with what very, very little we know of Rayse's. Why complicate the issue? Rayse==Odium explains everything; if it is wrong, it will have almost no dramatic impact because we simply don't know enough, and basically I just can't see the reason not to make the simple inference barring solid evidence against it in later books.
