Ok, so my theory is that Silverlight was formed by a group of Rosharan's aided by Khriss.
We know that Honor locked the Oathgates just before he died, which we know happened just after the Recreance. Prior to this time we know that the Oathgates were the source of much travel, to the extent that the Knights radiants imposed a tarriff through Urithiru. Up to this point I had been operating on the assumption that this simply meant that pre-recreance, the oathgates were used to allow trade between the silver kingdoms in the physical realm of Roshar. However, we also know for a fact that the Oathgates could be used to travel to Shadesmar before Honor locked them.
So, essentially we know that for the entire duration of the Silver Kingdoms era, every city on Roshar had direct access to the cognitive realm. Is it possible, that towards the end of this era, a large number of Rosharans emigrated to the Cognitive Realm, exploring outwards from Shadesmar, and built a city?
I would imagine this would likely have occurred sometime between the last desolation and the false desolation, the latter of which probably led to the loss of Silverlight from the cultural memory of Roshar. That rough timeline still allows for Khriss to have aided in the founding of the city if I'm judging it correctly.
If I'm missing something obvious that disproves this theory could someone please enlighten me haha, I fear there might be something obvious.