A couple of things come to mind, as I stay up waaay too late to read all of this:
Rashek probably took the power for himself because Kwaan had told him the truth - that the prophecies had been changed. Therefore, Rashek would be going on this mission to lead Alendi astray, knowing he wasn't really the Hero, and trying to dissuade the others from following him. And in the end, he could have even gone into the Well of Ascension in order to stop Alendi from doing it. And that would have been what the Mist Spirit wanted, so no resistance there. Created the things that would aid him in Preserving what he had set. He brought order. The Kandra could be spies, the Koloss troops, and the nobles the ones who actually believed him (and his uncle Kwaan), so they get Allomancy. All in the name of Preservation. Call him evil, but I think he was really just hardened by 1000 years of preserving things as he initially set them, which was influenced by the fact that he was going against pretty much everyone from his own civilization, since they all thought Alendi to be the Hero to save them all. And he was a cool guy from what we know, but possibly coming off arrogant since because he was so sure (outwardly) that he was the Hero.
If the thing trapped in the Well is Ruin, trapped by Preservation, then everything the Lord Ruler creates is for Preservation. Keep things as they are; crush rebellions, find your enemies through spies, etc.
The Ashmounts could actually be part of Ruin, since they seem to be semi dormant volcanoes, maybe even held dormant by the mists, which could be Preservation, depending on the kind of people the mists killed. Brandon does say in his annotations (Chapter 15)
Mostly, I'm showing the real danger of the mists--that there IS indeed a reason to fear them. Either way, remember one thing from this chapter. Some people were killed (and there's a connection between the two people you've heard described specifically as dying from the mists) some people got away, and some people had seizures, but then were all right later.
Which two people were "Old Jed," described as being a "hard worker" (pg 42). The other was "Old Jell" who was "Bullheaded" (pg 141). More thoughts on that?
Last thought for the moment: "Legends say" that there were no mistborns or mist before the ascension, according to Sazed, but we know that Copperminds are subject to Ruin's manipulations, so that may not even be true; a thousand years have gone by.
So, this doesn't account for the Mists coming in the daytime, and I know there's a lot I've missed, but there are my thoughts. Ideas? Discuss.