So I thought of an interesting visual. Imagine this: Snapping creates a conduit, a circular tube if you will, that allows you to channel the power of Preservation. Now imagine the metals as filters that can be placed over the tube, each metal having its own specific pattern, like the Aons. It is the specific pattern of the power coming through the filter that decides what power is being used. Mistings only have one specific filter, but Mistborn have all of them at their choosing.
Now, to the question of the Mists. Imagine that using the Mists burns through you more conduits, but this time instead of being circular, they are in the specific patterns for each metal. The more you draw, the more permanent these patterned conduits become.
I'm quite rough on theories of Realmatics, so I'll refrain from trying to draw parallels, but this seems to make sense.