You can enter Canada from USA just by CoPR only. The Canadian border officers will not prevent you. No PR card is needed.
http://www.canadavisa.com/canada-immigration-discussion-board/entry-to-canada-from-usa-without-pr-card-only-corp-t224502.0.html
The tricky thing is that there is a waterprint on the CoPR paper saying "not for travel", so the flight company will not regard CoPR as a visa to enter Canada. You may return to Canada by private car, or you may find a way to convince flight company that you can be admitted to Canada (study permit or work permit is enough).
If possible, please keep me updated. I probably need to go to USA for job interviews before I receive my PR card. Thank you!