Thanks for your test.
I thought the whole point is, that the shadows created by C-sun and or C-sky are not realistic when you build the room, leave an opening for light to enter and apply SC mat to wall floor and ceiling. See attachment. That shadow never changes regardless of what you use.
Just like your first post said, even when you make an “outer shell” with regular material that should just block the sun and/or sky light, it’s ignored and the shadow catcher receives full sun light.
My question is exactly for these “edge cases”, how to correctly light a scene where part of the furniture is in direct sun light and part in diffused bouncy light.
Did you ever get a better answer from the corona team?