Lets assume that the PLAN repeat the same action as the South Korean, they sent 100 well armed and well trained PLA marines into the island and set up defensive positions inside the island well digged in like these South Korea have done in the past.

The PLAN sent all their eastern force on alerts and along with Taiwan Navy... etc; everyone now on a Mexcian Standoff!  Both sides are armed to the teeth and China have their DF nuclear missles targets Japan island fully review to the American and Japanese people.

The next move is on Japan and the American!  What are they going to do?  Back down or seeing a full out shooting war and possible leading to the destruction of entire Japan?

What are these US and Japan politican going to do?  They are going to ask, how much lives and damages are these bare rock island worth to the life and economy of Japan?

You do the math!