I commend you for recognizing to dispute the sovereignty of Taiwan it helps to start with a definition. Unfortunately for you the definition you provided is vague and at ends with more formal definitions. I’ll reference you to the indisputable democratic source of knowledge wikipedia (feel free to edit the page if you it can be improved):

Sovereignty can generally be defined as supreme authority.[1] Sovereignty entails hierarchy within the state, as well as external autonomy for states.[2] In any state, sovereignty is assigned to the person, body or institution that has the ultimate authority over other people in order to establish a law or change existing laws.

The PRC and the USA do not pass and enforce laws in Taiwan. The Taiwan government, elected by the people of Taiwan does. They are self sovereign.

You’ve brought a lot of good points which I ought to go through in detail, but briefly: Vietnam great analysis but different country. Military - is Japan sovereign based on reliance on US? Are there only a handful of actually sovereign states (the superpowes) in your schema? Regarding not provoking PRC no shit they don’t want to get slaughtered. As has been pointed out they have organizations and relationships that are de facto diplomatic if they are not called that because of the gun to their head.

Curious, what’s your stance on Palestine’s sovereignty? I think they can be considered sovereign, I don’t see that spectre of other powers potential influence as taking that away. I don’t see why all you guys need to make the bar seem so high, if you individualize it this much the word changes its meaning. A nation doesn’t need to be uncontested among all other nations to be sovereign. If its not the Taiwan government who is sovereign there? Your position would require there be an “unsovereign” condition, unless you actually believe its the PRC sovereign there. Unless its contested within the borders I don’t see how you could make the argument a nation is unsoverneign.