If the process fails to deliver your wanted outcome then you have to abide to the rulings.
So if all Puerto Ricans unanimously decide to declare independence and the U.S. says “nah,” they’re just supposed to live with that? How is that just? You even acknowledge that’s the path to a revolution or civil war, which we can both agree is a terrible option. What right does any country have to impose its will (through violence, of course) on a unified region that wants to leave?
Once a region declares independence, why does it have to fight with one arm behind its back? Isn’t it free to seek out allies, as all warring countries have done throughout history?
Should the American Colonies have declared independence? Should they have sought the help of France to even the odds against their much stronger opponent?