>>>Explain how Congress' plenary power to "regulate commerce with foreign nations" is restricted by required 2/3 supermajority (the Treaty Clause).<<<
NAFTA is not a regulation of commerce by the congress, but rather an abdication of the authority to regulate commerce to cabinet-level representatives of the NAFTA member nations. There is no such authority under the constitution for the congress to abdicate authority, except by treaty, and then by 2/3rds majority vote.
You really need to think this through . . . if Congress has the ultimate authority, but that authority is governed by the 2/3 rule, which branch of government regulates commerce with foreign nations if a 2/3 vote fails?
In other words, if Congress has the ultimate, plenary authority, what happens when your inevitable vacuum occurs?