Good pics to explore. Those pics shows that the clouds reflects the sunlight in several layers. You can see greyish in the shadow side. I think it is more of 'Transcluence' effect. It is a good start to explore in transcluence effect for the realistic clouds. The first image posted in this thread. It looks more like fog to me. If you want clouds like the dual pics in post #6, then the volumetric should be more thicker, chunky, and reflective. Where did you get the shader from, maybe I can try to play with it to find varities for different realistic cloud effects.