I'm pretty sure that ashfalls also occur throughout the empire, am I correct? Just having ash and smoke in the sky would not turn plants brown, and all the ash on the ground should make the plants more healthy. I could be wrong, but I was fairly sure that places around volcanoes are very fertile because of the ash they produce.
The lack of wind is another interesting thing, so far in the books, the most we have seen is a light breeze, no violent windstorms or the like, it also rains very rarely from what I have seen, but that could be just because the central dominance is very arid and dry (besides the mist).
What if the Lord Ruler somehow aged the world (and universe or solar system or whatever) when he recreated the earth. Maybe the sun turned red because it became so old.
What if the ashmounts and the mists are both vehicles of Ruin. The only reason they are like Preservation is because of the constantscy. I still hold my theory of opposites, Ruin and PResrvation using eachothers opposites to trap eachother: Ruin trapped Preservation in the ever changing mist (before the mists were constant, remember, the mist spirit was around before the mist at the time of the deepness, and was in Terris, where the deepness had not reached yet), and Presrvation trapped Ruin using hte power of creation held at the well of acsension. Ruin created the afterward, to force someone to release him, and started al lthat prophecy stuff, and Preservation tried it's best to stop him.
I think the ashmounts are ruin's new vehicel for destruction, what's more destructive than a bunch of violent volcanoes and earthquakes?