You have hit on te exact problem why NOT formally importing the car is NOT a good solution.
First, I'm quite sure you did not take this TN job for only one year. Why would you disrupt your life so much just for that. Your hope is to make this a careeer. So, in a years time you will be needing to import the car anyways, why wait.
Second, TN is not like snowbird or even student, since the local state views you as a resident, which thus requires that you register and plate your car (and get DL) as a resident. In many states, plating requires that you demonstrate formal importation.
Third, and this is where you face the greatest risk, is that NO Cdn insurance firm will accept your existing policy as covering you when you LIVE in US. You are no longer visiting US, you live there and your insurance should reflect this.
Fourth, if you manage to get past all these barriers without importing, then you still have the problem of not being able to resale your car. You don't want to resale your car, you say? Fine. But what if you total it in an accident? Your US insuarnce would love to pay you off, however, you are not allowed to transfer the ownership to them, since you didn't import the car. Now what?
Get the letter from the Cdn maker, import the car formally, and you are done. It costs nothing.


