First, I have no experience with MGOPS. That said, I have used Black Gold compost because it is cheap and available to good effect.
Sand will always try to settle in my experience, but I think you are going about it the right way. I've done something similar and it worked fine. My suggestion is to do your egg crate lava rock thing, then add sand to fill in the gaps until the lava rock/egg crate is covered. Slope that as desired. Then add dirt and follow the contour of your slope. If you are worried, use Small pieces of lava rock or clear plastic at this point to make "damns" along the slope to prevent shifting. Top the dirt and damns with more sand and neaten up your slope. That will give you the best chance at a slope that won't sink or shift. If you use a carpet plant that has lots of roots like DHG or HC, you have even less to worry about when it comes to sliding or erosion.
Also, because MGOPS and Compost are so rich in organics, don't be afraid to cut your dirt with sand. I did this in a tank rescape (didn't feel like trying to separate the sand from my dirt) I took the old sand/dirt, mixed it with a little fresh compost to spruce it up, and I had a lot less trouble with algae in the second tank than the first.
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