I don't follow. If the U.S. breaches NAFTA, then what is Canada's remedy after adjudication? You would seem to suggest an equal and opposite breach, rather than enforcement/mandamus that a treaty might provide. Do I understand correctly? Retaliation is the example that Sen. Cruz gave yesterday.
NAFTA is written so that all parties understand that it is the US Congress that governs its current version subject to mutual intent and agreement. If Congress notifies the parties to the agreement that it intends to modify or sunset certain provisions, then a meeting is called to negotiate a revision. Everything is subject to US law.
If Canada were to declare a breach by the United States in the NAFTA agreement, they could file a suit for damages in a US federal court and the US Court after ascertaining appropriate jurisdiction would hear the case according to US law.
Treaties are regulated and governed wholly separate from US Courts. Breaches are usually subject to binding arbitration via a pre-specified venue that is written into the treaty. Damage awards are usually enforced via asset seizures and confiscations if a party is not forthcoming to honor the result of the binding arbitration.
The linked reference provided by the initiator of this thread has court case examples dealing with challenges to a trade agreement. One relevant paragraph is copied in post #14 above.