As far as I know you can cross the border on CoPR. However I will suggest you call CBSA and get confirmation. We called CBSA twice and both times the experience was good. All our questions were answered patiently. If you are used to US immigration experience, expect the opposite (and a very pleasant experience).
1. One of my friend provided me groceries. He left it outside my hotel room. The hotel staff was cordial too. They also offered to buy groceries if we give them the list. I think Walmart is providing home delivery so is Lobelaw's. I think my wife had used instacart too. Also Ubereats, doordash etc. are good to order prepared food/meals.
2. You can carry groceries from Michigan. We were not asked or stopped by Canadian custom regarding food /grocery etc. I think you will be ok to carry it. If you are crossing border in your car, I dont think they will check your luggage/car
3. You can apply for OHIP on day1 as the 3-month condition is waived off for now. But you cant visit Service Ontario until your 14 day quarantine is over. There is a number provided on OHIP site that you can call after entering into Canada. I did not call so not sure what assistance they provide. I went to Service ontario today (20 days after landing) and got OHIP. Process is very straight forward. Read this
https://www.ontario.ca/page/apply-ohip-and-get-health-card
4. My goods transfer is handled by my company so I didn't shop for it. You can use u-haul - that is cheapest. If you dont want to drive u-haul then try u-pack. When I was checking different moving companies, I felt u-pack had best and cheapest options. But I have no experience with it. So do your own research before you finalize