Exedus,

Unfortunately,

(a) as NAFTA subsumed and replaced the earlier US-Canada agreement, the better view is that if the US gave the requisite notice to Canada for termination of NAFTA, this would also terminate the provisions continued under NAFTA from the earlier agreement, and

(b) the only enforcement mechanism available under WTO is to impose countervailing tariff penalties against the US (and this would probably trigger a tariff war between a defiant Us and Canada).

In short, if the US were prepared to effectively ignore the WTO, then Canada only has two disadvantageous options: accept a US fait accompli or square off alone against the US in a trade war.