On the note of mists being entwined with all three magic systems, that really strucj me, we know it is attarcted to allomancy, and that Vin, at least, can use it to strengthen her allomancy, and that hemalurgy repels it, especially when it is being used: (IE, the mists only start pulling away from Vin when she is really listening to the well). Perhaps, it's more to do with Ruin and Preservation, maybe the mists ull away from things of ruin, and are attracted to preservation, and the reason Vin repels them, is because she is becoming more under ruin's nfluence at this point. We know know more than this though, how else do the mists interact with the magic systems, and how do they influence feruchemy. I have a hard time believeing the Feruchemy is JUST neutral and has nothing to do with the mists now, there has to be something more!
And about people who only use hemalurgy (sorry) we know that the inquisitors use SOME allomancy, because most of them are seekers to start out with, are they not. Marsh's recruitment seems to suppor this. If they could use anyone, why would they go to someone as new as marsh, even if he is hard working?