I just wanted to share my experience crossing from the U.S to Canada without a certificate of citizenship or a Canadian passport. I had my oath form, copy of my PR card, COPR with the line "Holder is no longer a permanent resident" written on it, my foreign passport, and my driver license.
Overall, it was not too bad. I was not sent into secondary inspection but it did take the officer a while. I think what might have helped me is that I know the officer and he's seen me a few times before. I cross at that border crossing very often. So you CAN go back to Canada from the U.S. through a land border without a certificate of citizenship or a Canadian passport. Your experience at the border might vary, however.