Wouldn't you say that any treaty signed at the end of hostilities is "essentially" signed at gunpoint? I don't think the Japanese, for example, were exactly thrilled to sign the 1945 treaty.
"If in 20 years the UN demands a "national sovereignty referendum" in the "Aztlan" portions of the Southwest (Kosovo-style) will you support it as legal under international law?"
In a word, "No".
What is the basis for a claim to return that territory to Mexico (or turn it into a Republica del Norte)? Just because a lot of Mexicans say so?
If the Mexican government wishes to scrap the Treaty of Guagalupe Hidilgo, fine. Give us back our $15 million and we'll just pick up the Mexican War where we left off. Hell, we'll just activate the Texas National Guard and let them do it.
This would have the advantage of pushing the border further south, making it that more difficult for the illegals. (Plus, Texas would regain their claim as the largest state.)