I am in a similar situation and have a few additional questions about landing and then coming back to the US right away:
1. Isn't a Canadian address required for them to send the PR card to? If somebody goes back to the US right away after landing, how will they be able to receive PR card?
Yes, a Canadian address is required. Most people give them a friend/relative/family member's address - CIC will mail them the card in ~2 months, and then they can FedEx it to you. If you don't know anyone at all in Canada that you trust to do this, you can still enter with no address. You have 180 days after entering to update the address, so spend those six months trying to find someone. If that's impossible, you can ask for a PRTD or cross a land border in a private car when you move for good.
2. If a Canadian address is required, will they accept a PO box address within Canada for sending the PR card? Or does it have to be an actual residence/street address?
Not sure. Someone else can chime in here.
3. I read on another thread that during landing, you have to present forms that show the value of all your goods which will be brought to Canada. If somebody is not planning to move to Canada immediately, are these forms still required?
Yes, they are. I just did this. You have to give the CBSA officer a list of all your good following (mine was a three page excel list + 18 pages of photographs for jewelry, 2 per page) and they will fill out a receipt and give it to you. When you bring your goods later, you show them the receipt and that list, and you won't have to pay customs duties.
Thank you in advance!