Whether you enter by plane or at the US border, it is possible that you will be reported on entry if they are aware that you don't meet the residency obligation. If that happens, you can appeal it and continue to live in Canada as a PR while your appeal is being processed which can take 1-2 years from what I've heard. Depending on your reasons for not having met the RO, you may win or lose the appeal. If you lose, you will lose your PR and be asked to leave. Time spent during the appeal will not count towards your RO unless you win. During the appeal processing, you can renew your PR card for 1 year at a time but it would be wise not to travel during the processing time as you want to show that you are ready and willing to settle in Canada at this point if it comes to that.
No Canadian law says that you must have a valid PR card while living in Canada. However, it's increasing that it's being asked for as a proof of your valid PR status. If you get a health card and drivers license before your PR card expires and stay in the same province for 2 years, you should be ok. In some cases employers may ask for it so it can affect your job hunting.
If you are able to enter Canada without getting reported, it would not be wise to try to renew your PR card before the 2 years have passed, unless you have some strong humane and compassionate grounds for why you were unable to meet the RO. Otherwise, you would only be attracting attention to yourself that you are in Canada and that you don't meet the RO and inviting them to investigate your status.