Canadian permanent resident/US citizen re-entering Canada with expired PR card

Boarding a plane isn't a problem for US citizens for whom an electronic travel authorization is not required.

Since you have an expired permanent resident card, your information should already be registered in the Global Case Management System so the officers could verify the status quite easily, especially with an expired PR card and a valid passport. Once your status is validly established, you have to be allowed entry into Canada.

Sections 11.2 and 11.3 of the operational guide for port of entry examination procedures have more information:

The PRC is the best evidence of permanent resident status in Canada.

The following documents are satisfactory indicators of permanent residence:

• the original Record of Landing;
• a certified true copy of a Record of Landing document issued by IRCC National Headquarters (NHQ);
• a letter issued by IRCC NHQ verifying permanent residence;
• a passport duly stamped showing the date on which permanent residence was granted if the person was granted permanent resident status before 1973;
• a Confirmation of Permanent Residence form [IMM 5292B]; and
• a permanent resident travel document.

...

BSOs at POEs have the discretion to authorize the entry of permanent residents, even in the absence of documentation. If documentary evidence is not available, the BSO at Immigration Secondary must establish the person’s permanent resident status by questioning the person and checking the person’s status in GCMS.

...

An expired PR card would still count as some sort of documentation, or you may still have the original landing form stapled onto your (perhaps old) passport. However, you may still face some questioning.

Once your PR status is established, you have the right to enter Canada.

after a short (say 1-week) trip to the US

Also this does not matter much. Review of admissibility and compliance with residency obligations can happen regardless if you have a valid PR card or not. A PR card is "shall-issue" and must be issued to a permanent resident even when the government has already initiated removal proceedings (as long as a final decision has not come into force after the appeal period has lapsed).

An expired PR card may be considered as a ground to suspect possible non-compliance of residency obligations. You have the burden to prove (regardless of the validity of your PR card) that you meet your residency obligations in the five-year period before the date of examination, although you can still enter Canada even if you refuse to answer questions regarding the compliance of your residency obligations (but the officer can continue their investigation and potentially make adverse inferences).