Dude thanks a lot for your post. I actually went today itself after seeing your post and came back.
Details:
Me on h1, wife in h4 . Both are stamps expired
Peace arch crossing.
The first Canadian agent that I passed through asked whether I would go back to US or stay in Canada after confirming pr. I said I'll turn back. Then I had to park on the stalls, they tookv my docs and in 20 mins was done. I was in car all the time. He told me how to turn around for US.
On US side, at the drive through check point I told agent that my stamps were expired but had valid i797.. Handed the same. She passed me to secondary inspection building. Again I had to stay in the car and an agent came and took papers. I was transparent and told them that I had gone for confirming my Canada PR.
I was done there in 20 mins.
I drove from Seattle to peace arch border.