Controlling sun light intensity and shadow in Rhino has been a long time wish of mine as well. Looks like @andy doesn’t see the need for this though - I don’t think this is on any To-Do list right now.
With the shadow intensity there is a loophole though - if you export the sun info to a file, the shadow intensity is part of it. If you edit it and re-import, it will work (not exposed to Rhino UI).
I wrote a script that automates this workaround - maybe this will help:
SetSunShadowIntensity.rvb (2.1 KB)
–jarek