I don't think Japan's economy can survive global sanctions by China, Korea, SE Asia, Europe, and US.
I also don't think US wants China to arm US enemies like Venezuela, Mexico, Cuba, Iran, Canada, Denmark with nuclear weapons, so it will not support arming China's enemies like Japan with nukes. It can backfire on US to support proliferation because China can play the same game too.
There will be no global sanctions, only Chinese sanctions. The "rules" apply on others and certainly will not apply on Japan if that means they add a complication for the "arch enemy" China.
China arming Venezuela is like handing over the nukes directly to US. Did we not witness the incompetence just rexently?
Mexico wouldnt even want it, their policy and actions is to get Chinese market share in US market. They arent even solid ally.
Same for Cuba, US will run circles around them, they simply dont have ideological fervor of the past. If Venezuela becomes a success story in terms of improving life of general public, Cubans will sit in homes. Your average person doesnt give a fuck about great power games.
Canada, Denmark?? Lets add Moon and Mars to the list.
Iran, TBH, too late and not worth riling up rest of the Middle East now. Whatever relative deterrence they had, that went out of the window. The remaining will be gone too as how things stand.
The best cards to play is Economic response, like we are beginning to see and perhaps build up of North Korea.
China is being decent and playing by the rules that are no longer being followed.