Canada isn't bound to the monarchy or to the UK. The king is just a figurehead, and the king's representative (the Governor General) is a powerless rubber stamp.

Now, to be clear, the (conceptual) figurehead is taken seriously. When the queen died, a great renaming happened, changing each province's court of queen's bench to the court of king's bench, Queen's Counsel are now called King's Counsel, and of course new currency is being minted with images of the king. But, none of that means the actual King Charles the person has any power at all.

What definition of treaty are you using that the USMCA is not a treaty?

The difference is negligible in international use, but in the US it means “agreement that was ratified by the Senate”.

Are you claiming the USMCA was not, in fact, ratified by the Senate?

Disputing under the terms of a treaty is quite different than unilaterally breaking a treaty.

Dairy quotas are part of USMCA.[1]

The issue under Biden was protectionist/preferential policies which were challenged and got changed through the dispute mechanism[2]. The changed policy was then disputed again but the US lost the dispute[3].

The US has never gotten close to surpassing the tariff-rate quotas so the tariffs haven't applied.[4] Though the American dairy industry claims that's because of further protectionist policies.[5]

I'm not an expert on this so if you have more specific information please share sources.

[1] https://ustr.gov/sites/default/files/files/agreements/FTA/US...

[2] https://ustr.gov/about-us/policy-offices/press-office/press-...

[3] https://ustr.gov/about-us/policy-offices/press-office/press-...

[4] https://www.yahoo.com/news/fact-check-trump-doesn-t-12005222...

[5] https://www.idfa.org/news/idfa-statement-on-potential-u-s-ta...