Thanks.
If you have examples where _Shell and _OffsetSrf fail, but FreeCAD’s Part::Thickness or Part::Offset succeed, please send them to me.
The Curves WB doesn’t seem to be accessible by the API without the GUI. If it was, BlendSurface could be applied after Part:Chamfer to create BlendEdge-like features.
In January, I started to work on an alternative to _Patch using FC’s Surface::Filling, but it is still a WIP.