Why does Isotrim and Domain2 recreate the base volume?

All components which evaluate a surface work on a surface, not the trimmed representation of a surface. Trimmed surfaces are not really trimmed in a sense, Rhino uses trim data to hide parts of the surface from you. There is always an underlying surface which is what you are seeing. There is no way to give a trimmed surface a domain as it relies on NURBS and NURBS surfaces are 4 sided objects. Same will happen for other components that use surface evaluation.