I suspect (and I believe other people do as well) that the Seons are Splinters of Aona's Shard, and the Skaze (which Brandon described in the annotations as "evil Seons" are Splinters of Skai's Shard.
I also believe that the spren are Splinters, as I have stated before.
Interesting observations about Shallan and Kaladin's mutual traumatic events. Remember that the reason for the Snapping of Allomancerrs is because people on Scadrial were made by Ruin and Peservation working together, so they have some of Ruin in them that needs to be overcome before they can access Peservation's power. We don't know if other magic users on other words would have the same issue cause we don't know how they were made.
and now I have to go wash dishes, maybe I'll think of some more stuff to write later.