Trade with the US would not cease without NAFTA - all that would happen is the reimplementation of tariffs some of which since the FTA in the 1980's have moved to zero, the for most the highest is 6 % under agreements via the WTO.

Production is not going to move back to Canada and the US in any major form.  As already stated by the auto manufactures if the US content and NA content rules go thru they will ship the cars under WTO rules of a 2.5% duty instead of Nafta.

As for the balance of trade - the US will always as an economy import more than it exports.  With the exception of China and India it has more people than anyone else it trades with.  A country 1/10th the size will usually import less than it's neighbour.