Thought experiment: if say Rhode Island had a Muslim majority and they voted themselves Sharia law in violation of federal articles, how cavalier would the federal government / the rest of the populous be in tolerating this?
They gave it back with the condition that HK would remain quasi-independent until 2050. The people are protesting because they see the writing on the wall and realize independence is fleeting.
Without getting into the specifics of Sharia law, I think we can reduce your next question to "how cavalier would the federal government / the rest of the populous be in tolerating a state or city that violates federal law?"
* Northern states endured slavery for dozens of years while passing their own laws that criminalized slavery, allowed slaves to buy their own freedom, etc. (however the South responded by passing a federal law that required Northern states to capture escaped slaves and bring them back, one of the precursors to the Civil War and the only time the system totally failed)
* Prohibition failed so hard the Fed actually gave up and nixed it from the Constitution
* More than 30 states have decriminalized marijuana going totally against the DEA, a federal agency with certain powers defined by federal law
* Many cities are "sanctuary cities" which means they refuse to cooperate with ICE and DHS, again federal agencies
* Some states, especially Southern ones, routinely encode new strategies into law to bypass Roe v. Wade, which was a decision made by the third branch of federal government, the Supreme Court
* Upon the passage of the ACA ("Obamacare"), multiple states immediately sued the federal government and refused to implement sections of the law
While these actions certainly cause tensions between individual states and the Fed as a whole, the Union still holds together due to of the lack of authoritarianism. Because of the decentralized nature of Western power (as implemented via federalism and other strategies), it is impossible for one figurehead or political party to dictate what happens in every corner of the country. Entire legislatures can be overturned by citizens who wish to ignore the federal government's worst wishes. This is a feature - not a bug.
That being said, there are limits to the tolerance, and breaking the Constitution is one of them. Would people support a state ignoring the Bill of Rights? No, absolutely not. But that's a bad analogy because the HK citizenry aren't trying to take away others' rights; they're trying to give themselves more. Would Americans support a state that fights for more rights for the people? I suppose it's subjective, but I would say yes, yes they would support a state that aims to give more freedom to the people. That difference is crucial.
>if say Rhode Island had a Muslim majority and they voted themselves Sharia law
This question hinges on the premise that a majority of people in a Western democracy would decide to willingly give up their rights to the state in recognition of a greater power. Are there groups like this? Certainly - evangelicals, Wahhabists, etc. Do they make up a majority? I really doubt it. I am hard pressed to think of any situations in history where any large mass of people have decided to replace their democratic freedoms with a dictatorship or theocracy. Authoritarianism is usually a result of a violent takeover, hence why most dictators live and die by their armies and not their citizens. So the question is a false premise in my eyes - why would any majority vote their right to vote away?
The government of China pooh-poohs this treaty as a "historical document" and says it basically doesn't apply any more.
That said, the idea that "Hong Kong is China's property" would at least pay homage to the ideal of the Westphalian nation-state, a concept which China has also officially declared as an outmoded concept, and they have made it clear at all levels that they enjoy meddling in other countries' affairs. (Of course, while they are eager to dish it out, they can't really take it, and there is no end to the hypocritical caterwauling about foreign interference, especially regarding Hong Kong...)