they also import a lot of energy like coal from Australia

In particular the US could declare a blockade and drop a handful of naval mines near Chinese ports. At that point almost all neutral merchant ships would have to avoid those ports because it's virtually impossible to obtain insurance in conflict zones, especially because insurers can't risk violating US financial sanctions.

The blockade is a war act. If China starts the war then sure. Otherwise blockading China by "dropping naval mines near their ports" is sci-fi area.

Yes, of course. Declaring a blockade and mining harbors would only happen as a response to some sort of major aggressive act by China. It would be an act of war, but still a step below direct kinetic attacks on the Chinese military. It's possible to de-escalate from a blockade.

>"It's possible to de-escalate from a blockade."

How? Oh please remove the blockade, we'll be a good girl from here on? Yeah sure. You think they would even let it get to the stage when harbors are mined?

>"Can they protect the sea supply lines from the US navy?"

The US will not attack China's ships (assuming China did not start the war). Doing this is straight WWIII scenario. All bets are off in this case. You think the US is suicidal?