correct, generally speaking the COPR expires the same day medicals do. if medicals expire really close to DM, more than likely CIC will extend them and give an expiration date for weeks or months later.
Yes, you can do a "short landing". Meaning, you can land, return home to pack up, and come back to canada. When you do your landing, you can fill out the "B4" (recently renumbered) form and fill it out as "goods to follow". When you come back with your stuff, you can settle your goods then. there is no time limit for when you can bring your stuff into canada (from what i've been told), so you don't have to bring it all at once. just make sure it's listed as goods to follow.
Keep in mind your PR card will be sent to your new canadian address, and it could take an extended period of time to get it after landing. For US citizens though, it's not required to have it to travel. your stamped COPR and Passport will be sufficient enough to enter canada until you get your card.