>I haven't looked at the details of EclipseGrid. Is that a fully formed (compressed) grid or some intermediate structure that includes the complete corner-point description in an accessible way? 

It is a thin handle holding an ERT `ecl_grid_type` instance. That is a complete corner point description containing cell centers, cell volumes and all corner coordinates. In addition ECLIPSE peculiarities like coarse cells, LGRs and dual porosity is supported. Cell-face relationships is not explicitly handled in this datastructure.