I would like to add my experience. On this Tuesday I went to US export office from Canada to formally export my spouse's car ( by the way still US plated/insured/registered after her landing in August 2018, almost 2 years ago) with proper ITN number and 72 hours notification. The CBS officer refused to allow me ( the spouse) to export my wife's car and requested Power of Attorney document. So, I said that I'll bring the document "tomorrow" and proceeded to Canadian side. There I declared (at the booth) that I am driving to Canada to import my spouse's vehicle ( with some more questions about myself) and at the CBSA office did the formal import using her BSF186 form with the vehicle VIN listed there. Why it took so long to import from the time of landing? There was a Mercedes re-call, after that this virus situation, etc... So, there are no any duties and with issued import Form 1 I proceed to our home in Canada. The same day I paid my RIV fees, sand them a copy of Form 1 ( from CBSA) and at night got an electronic RIV Inspection form.I understand of what you are saying, but I just not sure how Province rules work with formally imported car as far as how soon you have to actually register the imported vehicle.
Here is what we did. My spouse landed with vehicle (year/make/model/vin) listed on List of Goods. We did not do the formal export at US export office and did not do formal import during the landing. At the time of landing we told the CBSA that the import will be done "later", since there was an active re-call at that time. So, we were using the US plated/insured/registered car in Ontario for about 2 months. After that we did all the steps to Export ( US) and Import (Canada) and registration after that.
Yesterday, on Wednesday, I drove again to the US Export office, this time with Power of Attorney ( and clear Title) and did the formal US Export ( stamped Title) and again proceeded to Canadian side.
Here is the interesting part of entering Canada on US plated (exported and imported) car. I'll skip the part that I explained at the booth that I am dual citizen coming back home from work ( I work in US, so no 14 days self isolation for me ). The question was about the vehicle. I just presented the CBSA officer their Import Form 1 and explained that I just started RIV import process. He said that I can drive like that back and forth, but no more than 45 days after start of RIV Inspection process. So, my impression is that after 45 days they ( Canadians) would expect me to have the vehicle Canadian plated/insured.