I do not think that the rock background needs to be pushed back more into the dark, because you very cleverly shaded all the edges of them which has the effect that the brain automatically sorts them in 3D space for you.
However, the vines in the background are only shaded at the very top, and if you don't focus your vision in that location, you may be fooled into thinking that it's in the foreground (until you see the foreground vine).
As for the dithering, I love it. It makes it feel very crisp and detailed on my display (Macbook Pro).