Indeed, for shadow volumes, the shadow casters must be 2-manifold. We have some pointers how to check it, and help it, on Shadow Volumes | Manual | Castle Game Engine .
Note that we also have shadow maps. Right now they are uncomfortable to set up, require to rearrange your data. But I plan to improve them a lot and then using them should be trivial. Shadow maps do not require geometry to be 2-manifold. And you can have multiple shadow maps, i.e. multiple light sources casting shadows. Once I improve shadow maps, they shall be much better than shadow volumes, stay tuned 
One thread where I’m gently nudged to “fix shadow maps” is here Shadow maps in 2026? - #6 by michalis , you can watch it and ping me about it
Not exactly. Our shadows (both shadow volumes and shadow maps) follow reality in this regard: shadow is not something “where we draw blackness”. Shadow is the absence of lighting. You must illuminate the object, to have “something to turn off” within the shadow.
We have one internal capability that is close to it: one can make some light not being accounted into calculation if another light source is in fact in shadow. This is not exposed yet though on the TCastleXxxLight components, as I admittedly didn’t find enough use-case for it.
I’m interested in the usage you plan, please feel welcome to expand more about how you want to use shadows Maybe I can help advise how to set it up with our current system, or it will influence my plans how to improve shadow maps:)