I think establishing a blockade is generally considered an act of war. Possibly a war crime if the intent is to starve civilians? (Not sure, this is way outside my wheelhouse). I’d expect it to be about as escalatory as using US ships to attack the mainland, more or less.

Not a war crime, China has plenty of agriculture.

See the Cuban missile crisis. It is an act of war but certainly not as escalatory as direct attack.

Chinese agriculture is heavily dependent on fertilizer and energy imports. A blockade would easily result in famine.

In fact the history of China is just constant famines.

Is the history of China any more famine stricken than any other region if the world? In recent history (last two centuries) maybe, but before that? Typically you don't become the worlds most populous country by having frequent famines.

US action following a Taiwan invasion could be shoot a few missiles at the invasion fleet.

Declare all coastal areas around China a conflict zone.

Declare anyone trading with China can't trade with the US. If backed by allies, China could perhaps be boxed in.

But, hey, who knows. It's entirely unclear what is possible.

And much depends on whether Taiwan falls in 3 days, or stays in the fight for 3 years.

Or so one would assume.. current administration is proving that they'll abandon freedom at minimal cost.