Wait, wait. Why do you have to use the sun? I understand the spotlight can't be shining on the camera, but can't you just move the camera out of the light's cone? The only way I know to control contrast is to fiddle with the [Ambient vrs Sunlight] and the ambient [Uniform vrs. Sunlight] settings. Oh, and the general [Dark -- Light] slider. But heck, you already know about those. I have no idea yet how those settings affect things in a volumetric atmosphere. I put a scene together today and tried out a volumetric atmosphere for the heck of it; all the shadows/contrast remained the same, but the sky washed out badly, so I switched back. Still, the contrasts didn't seem to change from it, so maybe those settings still work the old-fashioned charm. :)