I agree with much of what you said but the point about automobile manufacturing is simply not accurate. Those manufacturers you cite are actually separate Canadian companies. For example, there's GM Canada, Ford Motor Company of Canada, and Honda Canada Inc.. Separate websites, CEO's, staffing, accounting reports, etc.. They open, close, consolidate, and switch assembly and manufacturing sites internationally on a regular basis. For example, GM Europe and GM Australia used to build and ship certain models to the US - not so much anymore.
Nissan moved assembly of the Sentra from Tennessee to Mexico in 2000. GM moved assembly of Silverado crew cab model pickups from Detroit to Mexico several years ago. Ford switched production of the Fusion to Mexico in 2005. Dodge assembles the 1500 series of Ram pickup trucks in Mexico - that happened a few years ago. That's about 500,000 pickup trucks annually that used to be assembled in the United States but are now assembled in Mexico.
From a pure procedural standpoint placing a tariff on General Motors Canada exports to the United States is not at all difficult.
Now, obviously it is in both Canada's and the United State's best interest to come to an agreement and I would like to see that happen. And personally the Canadian Supply Management System gets quite a bit of criticism from the US - where we subsidize farmers as well so IMO that's a bit hypocritical although the US farm subsidies are not as exclusionary and hyper-protectionist as the Canadian system as far as I can tell from reading up on it. There's certainly more free market forces at play in the US dairy industry as compared to the Canadian dairy industry it's fair to say.
But you are mistaken to think that GM and Ford brand and badged vehicles assembled in Canada and auto parts manufactured in Canada and exported to the US are protected from US tariffs.