Authoritarianism is easy to impose and democracy is fragile; the civic discourse required for a strong and stable democracy relies upon a considerable amount of mutual respect and rhetorical restraint.
This is false, though it's a popular modern myth; the whole reason for the Constitutional Convention was to strengthen the government under the Articles of Confederation to address problems caused by it's weakness, and the framers decided they needed to go a lot further than minor tweaks in doing that, sure some of them wanted to go even farther—Hamilton proposed an elective monarchy, for instance—but even the more modest final outcome was neither in intent or fact “as weak as possible.”
A stable, prosperous, democratic China is ideal. An easy way for an authoritarian government to keep internal dissent down is to make an external enemy. Think of all the examples you mentioned. So this is probably bad news for the U.S. and the whole world.
Neither has democracy, depending on the definition of "worked". Try to narrow it down.
> The Constitution was designed to make the US Government as weak as possible
That is a premise, not a truism. Every democracy has, in time, given in to an appeal by the masses to (either by dupe or passively) give the government more power until it's not relatively weak.
China has quarrels with almost every nation that surrounds it. Even nations too far way to be affected by the sabre rattling are starting to question China and their influence (ie the debate in Australia right now).