Update: the northbound experience
Entered Canada with cat today via I-87 checkpoint. No wait, pulled right up to the booth and handed the Canadian official 2 passports and the cat's paperwork. She pulled out the vet forms and looked at me as if she was puzzled, so I said "Those are our cat's proof of vaccination; we have our cat with us."
Then we got the official glazed eyes look, the one that says "I have no idea with I'm supposed to do about this, so I'll ignore it."
She then asked me multiple questions about guns, alcohol, tobacco and pepper spray, none of which we were carrying. Her final question was to ask if I own the car I was driving, and then we were on our way. Not a single question about the cat and she certainly didn't read his paperwork.
I'll post again after we return to the US about our experiences re-entering the US with the cat.