It's still pretty tenuous, but I suppose it's the best we've got. It just seems so hard to know where the Shard's referenced were, given that both Hoid and the recipient already know the background information.
Definitely. And complicating matters is the fact that--if Odium went to Sel prior to the events of Elantris--we don't really know what "Splintered" means, which makes it even harder to discern what Shard does what on that world.
Actually, I was really resisting this theory. I wanted there to be only one Shard on Sel. And then I re-read Hoid's letter (yes, I'm certain Hoid was the author) and realized that yeah, there were two Shard's in Sel. One was Aona and the other was Skai. That's by far the best reading of the text. Odium's visit was probably quite eventful. However, I actually believe it occurred well before the events of Elantris, at least most of it.
Why? Because I believe that the Aons and the Skaze are the splinters that Hoid talks about. The splinters of the original Shards. Think about it: Nobody knows how they are made. They seem to be immortal (baring some odd conditions that appear only in the deleted scenes). They are conscious---I can't emphasize that enough. In Mistborn, in order to make humans self-aware, Preservation had to sacrifice part of himself, but these Seons just float around conscious as though it was nothing special. And in some sense, they seem to embody an ideal. Kind of like---a shard. The Seons would be shattered Aona and the Skaze would be shattered Skai.
If I'm correct, then the reason nobody knows how to make Seons is that you can't. They just exist and get passed down because they've existed ever since Odium's visit. They have Aons at their center because that is the nature of the power they represent, the power that was shattered.
Anyway, it probably has some holes. Interesting, anyway.