I'm not sure this is possible in an actually smooth and realistic manner.
There are a lot of anomalies that look like bugs to me, maybe
@Matt can chime in.
For one, a continuing problem with offset/child displacement is the smooth zone of that displacement isn't coloured by the child/colour input, but is of the underlying shaders. Big issue with any of this type of snow displacement boundary stuff.
And two, for some reason the snow isn't following the slope constrictions, and on all of the stone. Smoother colour and displacement zones don't help. Patch sizes only make it rougher or polygony.
Here's a example file where I gave up.
----
One way it "sorta" works, but is just as ugly, is doing the compute normal
after the stones merge group, and than using masks to colour, and add snow, but the mask boundaries cause anomalies at the base of the stones, and need smoothing, which I'm not sure how to do when the stones move along displacement of terrain. Only way I know how is displacement to scalar and adjusting it, but this includes displacement underneath (as despite preview looks at the whole node tree at render time).