Of the people that were sick for the 16 days in comparison to just the one day, it is mentioned that they would be able to burn more precious metals (atium). Could it also be possible they are/were Mistborn—with the ability to burn all 16 metals?
Well, what was going on here was a clue established and set by Leras before he died. He wanted something to indicate—should he be unable to inform mankind—that what was happening wasn't natural, but instead something intentional. He worried that men wouldn't be able to realize they were being made into Allomancers.
And so, the mist was set to do something very specific, as has to do with the interaction between the human soul, Allomancy, and the sixteen metals.
Each of the 'Shardworlds' I've written in (Mistborn, Elantris, Warbreaker, Way of Kings) exists with the same cosmology. All things exist on three realms—the spiritual, the cognitive, and the physical. What's going on here is an interaction between the three realms. I don't want to bore you with my made up philosophy, but I do have a cohesive metaphysical reasoning for how my worlds and magic works. And there is a single plane of existence—called Shadesmar, the Cognative Realm—which connects them all.
You will never need to know any of this to read and enjoy my books, but there is an overarching story behind all of them, going on in the background. Adonalsium, Hoid, the origin of Ati, Leras, the Dor, and the Voice (from Warbreaker) are all tied up in this.