Only the american side needs to be informed, it is because you are exporting the car out.
In fact if you never plan on importing it back in you don't have to stop at all, nobody will check on the Canadian side.
The car does not have to be in your name on the Canadian side, you need the current registration signed over to you, and a bill of sale.
I did not register mine as I had it on a trailer. Why don't you have it shipped up here, there are companies which will ship it here. They will take care of all of the headaches for you
Absolutely no issue declaring it on the canadian side with the current title and the bill of sale.
The bigger part that you have not talked about is the recall letter. This will give you no end of grief. Get the dealer on the US side to give you the current recall status or you will be paying BMW canada huge for this.