So what you're saying is that I was right, all the the SC would have to do is change their ruling on the 2nd, then it would be "constitutional". Also I'm right that the executive could do this and it would later need to be challenged. Thanks.
Yes, an alternative route to gun control would be to strike down the numerous rulings on 2nd Amendment rights and
then pass this international agreement. However:
1. If these rulings are reversed, then the international agreement is redundant, because our 2nd Amendment rights will already have been effectively curtailed by the reversal of the rulings under consideration. If that is done then the treaty is irrelevant anyways because it will become possible for the government to establish domestic gun control laws without having to resort to treaties.
2. There are a big cluster of rulings that would have to be reversed, and the chances of the S.C. suddenly going on a streak of reversing over a century of case law is essentially zero, and
3. Since the case reversals in (2) aren't going to happen before this treaty emerges (if they happen at all), they have no bearing on it. The treaty isn't getting passed if it conflicts with the Constitution, and even if it did it would get challenged and struck down in a heartbeat.
Since I'm only focusing on what I think is humanly possible rather than possible in theory, I dismissed what you said. But it is definitely possible in theory, and further, you are definitely right that the treaty could be put in place by the executive (with the consent of the Senate) even if it was inconsistent with the Constitution; it would have to be struck down later.