(Ignore my bad English, but it's because I speak Portuguese so I'm using google translator to speak)
I made an example in this video with skysphere and some enemies walking around. your engine is really cool. even if a lot is done manually it is really good. I was using a separate layer and a duplicate of rojo3d in the same layout, then I could leave a layer just for the skysphere and another one in front for the whole scene. I noticed that the higher the draw distance, the more the shadow gets buggy and disappearing when I went to test it with the skysphere. then separating the layers and duplicating the rojo3d, I could put the short draw distance in the scene, and the skysphere one with a very high draw distance.
see the example video: