> Do you think Monaco should be part of France?

Monaco is already 90% part of France. There was an agreement until recently that Monaco would become French if the Princeship went extinct. By law the Prime Minister and the Police has to be French. France also handles their defense etc. It's very conditional sovereignty, the deal being that they can be a tax heaven if they want to, but not to France and Italy.

> Do you think Singapore should be part of Malaysia?

AFAIK they've been expelled from Malaysia after independence.

I'm not trying to disprove your point, just that it's fluid and fragile. Sovereignty itself has only been conceptually defined with the Treaties of Westphalia, it's recent and quintessentially Western.