China would have to destroy Taiwan before being able to set foot on it. It's not like Ukraine, there's no massing troops on borders. It's small beaches and massive cliffs.
What does that have to do with whether the US would help? Their interests are in the sea rights in that area, not just with Tiwan as a trading partner.
I think if an invasion was coming, us troops would be stationed in Taiwan as a deterrent, so that if they were killed there would be a lot of political will to fight back. And once an invasion starts, the US would still want to keep that part of the world in it's sphere of influence.
Are you saying China won't invade? I tend to agree with you, and I definitely hope you're right. But I don't see why the US wouldn't intervene if there was an invasion.
As for one China, that position is strategically ambiguous to create the least change of an invasion. See here:
https://youtu.be/809H6hy37nI