Exciting scenario. However imagine the Japannese exercise their capabilities and do what you say and badly bloody China. Do you suppose that those actions would not provoke a huge power struggle in China with the humiliated military demanding reprisal. And what if that military decides to respond with tactical nuclear weapons? Japan which is “under the American nuclear umbrella” would expect and demand an American in kind response. The American “nuclear umbrella” when designed and implemented was when China was weak and largely without nuclear weapons or the means to launch them against the United States. These “conflicts” in Asia can easily escalate. The United States has a duty to its own citizens to reexamine commitments, political and military involvement in Asian affairs. Douglass MacArthur himself warned that one day America would be in this difficulty.
Give the rising Sun Nukes, this time with delivery systems that can reach China.