There are different scenarios. I lived in Canada, but drove back and forth to my US office to work on my US plated / insured car for a while and finally got tired to explain every day US customs and Canadian customs where I resident/visitor, etc. So after several months I exported car and registered in Ontario and all these questions gone - all is simple: the resident of Ontario (Canadian and US citizen) goes to workplace in US and returns back on his Canada plated/insured vehicle.
1. When I did registration of my vehicle in Ontario I surrender the US Title and got back their green reg card, the ServiceOntario did not pay attention to export stamp in the Title.
2. If you formally export the vehicle and later return to US you can re-import the same vehicle back to US for free and to restore the Title.
3. Today the vehicle located in Canada and US Plated/Insured - are you sure you are covered by US insurance and will be reimbursed in the case of accident?
4. I do not agree with "...but the car has to be in the US for 72 hours prior to the export process...". At least at Detroit border. I sent email about my intend to export for 72 hours prior the export and drove to US office from Canada on the actual export day ( did it twice - with two cars).
5. Forgot about your US DL. I think in order to get Canadian insurance you have to get first (exchange) the Canadian DL and surrender your US DL.
6. I was using
https://simplifiedtradesolutions.com/ to get ITN number ( $35 and the next busines day).