I don't understand why you're still messing around with a mixed material when it's not  giving you the result you want.  If you'd  follow my suggestion, the enviroment settings of the Layer would give you the effect you wish to achieve.  You can layer mixed materials, but for the effect you want to achieve with the snow, the controls of the Enviroment setting are better suited for it.  Here's an example of using the layers. The snow and rock are single materials the grass is a mixed material. Mixed materials are nice for certain effects, but so is the layer function. But this is just my opinion and how I would approach this problem.