US should support a Singapore style outcome as the communists have breached the agreement.
Vehicle should be the UN. If the UN is good for anything it’s this. The treaty to “ return” Hong Kong is an international treaty recorded with the UN. Treaty has been breached.
UN Charter and position on colonial territories is that colonial holdings have the right of self determination, which Hong Kong was never given.
The only part that was a colony was HK island and a tiny part of the mainland.
Any fight at the UN would be for an island only.