OK, after reading Brandon's newest annotation, I'm pretty sure the mist spirit is very connected to either Ruin or Preservation. Brandon makes a note that the Spirit's Bronze pulses are the same as the pulses of the well. Well, if the pulses of the well are from Ruin (assuming that only those with hemalurgy can hear them), then what is the Mist Spirit. As of yet, we have heard no deepness pulses, so is the Mist Spirit more than just Preservation's mist servant?
Rereading Well of Acension also got me thinking. You know the part with Sazed's Lock and Key speech? Well, what if this also applies here. For example, the deepness, and the well seem like opposites at first, they oppose eachother, until we find out the the deepness was really jsut a means of making the world desperate enough to release Ruin from the well: lock and Key. What if there is something similar going on with the Msit spirit and Preservation, or even Preservation and Ruin?