On the issue of those spots in the sand. First of all, I don't bother with AO in a scene like this. There is only a tiny amount of IBL - most of the light should come from a single infinite light. So ambient shadows are largely irrelevent. Second, even with AO off, there is still a shadow bias problem with surfaces displaced by the cellular node. Try raising the shadow min bias on your infinite light (the one simulating the sun). Mine was set to 2. Below 1, I get the same artifacts as you did, even without AO.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)