Hi Tim! The scene is looking cool, I like the elegant straightforwardness of it.
Going by the older scene shots, I would say your diffuse is too bright. Even though you are going for a very white-colored rock surface, consider damping the brightness on your diffuse texture so that more subtlety in the lighting can show through. Right now the contrast between lit and shadowed surfaces is so intense that it's hard to make out any surface definition within either zone relative to itself. Bringing the diffuse brightness down a notch should help make lighting differences more apparent.
That said, also consider placing a few faint point lights yourself by hand to reinforce the idea of bounced light and to get more gradation along surfaces -- maybe pick a few of the brightest or most orthogonal surfaces to the sunlight and put a soft point light in the middle of them. To balance out the addition of these new lights, consider dropping the Lightmass bounce light intensity a bit to compensate (I usually do this through the DiffuseBoost property in the View>World Properties>Lightmass). Here's a quick paintover to show this idea: (this is an exaggeration to make it more clear)
Also consider toning down (if not removing) the SSAO as it's still a bit overpowering for the lit areas. Maybe try using the baked Lightmass AO option instead since you can control how much it affects lit and shadowed areas separately.
The surfaces on some of your rocks look a bit mushy. Not too much, but they have a fairly soft appearance that I think comes from not fine-tuning your rock phototexture to match the sculpted normals. Rather than resculpting and rebaking them, you might try painting more into the diffuse to reinforce the shapes in the normals. Maybe bake out an AO map from the sculpt with a significantly higher occlusion exponent (for mentalray bakes in Max, this is the Spread property) to better catch the pockets and recesses in the surface, like this:
And then using that new AO as a guide, and disabling your rock photosource layer so you can see more clearly, trace the edges of the pockets to pop them out further. You may want to trace over a faint overlay of your normal map as well so you can catch sharp outer edges too which won't have any AO contrast to guide you.
Here's the process I use to paint up my rock diffuse:
Lastly, and unfortunately I don't have any good examples of this from my own work yet, consider using multiply decals to discolor your rock surfaces as in from water damage. Doing this with decals makes it easy to avoid the "these two rocks have identical stains" problem. I'm still working on figuring out a good solution for moss/lichen myself, although I can show you how to set up a biased blend vertex paint shader for it if you like.
Hope this helps!