If memory serves, Marshall in Marbury v. Madison wrote "If a state is going to execute a man on the basis of an ex-post facto law, is the court supposed to let the execution take place?"

Article I Sections 9 and 10 explicity prohibit passage of ex-post facto laws by the US Congress and State legislatures.

In one sense, the supremacy clause places the constitution, legislation and treaties on the same level.  Loosely:

constitution = supreme law

legislation = supreme law

treaties = supreme law

Marshall goes on to state that the order of precedence is in the order of presentation.  Constitution trumps legislation (and treaties) when it clearly applies.  Almost certainly, Marshall intended this.  Did he also intend that legislation trumps treaties?  As written . . .

If the US constitution's proscription concerning ex-post facto  law can be applied against state legislation, could not its protection expressed in the 2nd amendment override such a treaty?

One weakness in the argument presented above is that the supremacy clause declares legislation passed in pursuance of the constitution is the supreme law of the land.  Based on the phrasing of the supremacy clause, I'm not sure if treaties have to be in pursuance of the constitution.

The SCOTUS has issued some surprising rulings recently.  If they failed to uphold the second amendment against such a treaty, I don't know if I would view that as a surprise or not.

Jefferson warned that intepretation of the constitution should not be left to the judiciary.  If the USoA is ever again (assuming it ever was) a Republic (not to be confused with a major political party with a similar sounding name), the people must know the constitution and insist upon its observance by state legislatures, the US Congress, the judiciary and other public officials at all locations.

Is it at all possible that we are hopelessly screwed?!

DC Treybil