Okay, try this: change the last line of find_dual_cell to
return SimplicialComplex(dual_cell_facets)

• I guess then you can delete the function find_cofaces and the one call to it in find_dual_cell.
• Would it help to be able to do sage: K.face_poset(options...) with options including facade=True? That would be an easy thing to add to Sage.