It's fun to war game. Even if you're a Grunt.
Methinks the Japanese have both fuel and design for a nuke. An outlier in the above game maybe the Japanese put one together in 6 weeks, amend the Constitution, and test.
My bet is the Chicoms and Norks (not in the above scenario either), would sh#t their collective pants.
5.56mm
Or, if we had a real POTUS, we would lease a boomer w/ nuke warheads to Japan and then airlift a Japanese captain to the boomer to give orders.
That would keep China busy for a while, especially if we told everyone that we had leased three boomers (where are the other two???)