A Permanent Resident of Canada generally must be allowed entry into Canada. The problem with not having a currently valid PR card is getting to a Canadian POE, since commercial transportation is required to check all passengers on Canada-bound flights for authorization to enter Canada before boarding. Thus, not having a currently valid PR card means the PR should not be allowed to board a flight destined to Canada.
But once a PR reaches a POE, a PR must be allowed to enter. As you apprehend, then, arriving at a land crossing by private transportation is a way a Canadian PR not in possession of a currently valid PR card can reach a Canadian POE without having to apply for and obtain a PR Travel Document.
Ordinarily proof of identity together with presentation of the landing document should suffice to establish one is a Canadian PR.
That said, a person outside Canada not in possession of a currently valid PR card is presumed to not be a PR, to not have valid PR status. So there are high odds there will be a residency examination at the POE.
I would suspect that the details of residency revealed in the citizenship application process could be a factor in how the residency examination goes.
In any event, best to be prepared to prove you have met the Canadian PR Residency Obligation, 730 days within the immediately preceding five years, when you arrive at the POE. Have your documentation in your hands (to carry into the office for the interview in secondary); do not leave it in the vehicle.
Of course you may not even be referred inside for an interview in secondary. How it goes at the Primary Inspection Line (the booth usually when driving across from the U.S.) could depend on what is reflected in FOSS. And likewise, how thorough an interview CBSA conducts in secondary, assuming you are referred inside for a secondary interview, may depend on what notes there are in FOSS. Still, again, be prepared to prove you have complied with the PR residency obligation.
Reminder: the relevant five year time period will be determined by the day you approach the POE. You should be prepared to prove you were present in Canada for at least 730 days in the five years immediately preceding that day.
