I couldn't help but notice something on my second Elantris readthrough. The Elantrian books Roaden studies mention the the Dor seeks only to escape, and that there is immense pressure in its realm, forcing it outward. We know the Cognitive and Spiritual relams are very closely linked, so it makes sense that that immense pressure would have an effect on the Shadesmar. Maybe worldhoppers just get ejected back out (And like the travel Aon, who knows to where?), or maybe something else. Either way, I think the Dor and how it relates to Aona's shattering is crucial to understanding Sel's cognitive realm.