Your parents will most likely be allowed in with an advise to meet RO by CBSA at the land border. Better for them to complete RO before leaving Canada as CBSA May not allow multiple entries on expired PR.
I had applied for PR card renewal on H&C, IRCC didn’t respond for two years. I had the US visa so came via land border and they allowed me in. Your best chance is to come
via land border provided you manage to get US visa.