How could China stop a nuclear weapon-capable Japan from interfering with AR of Taiwan?
As it is, a conventional power-Japan is already a formidable force to reckon with, imagine a nuclear Japan?
Japan is stockpiling enough Plutonium to make 5500 warheads, that's a fact.
Japan is known to take gambles against better strategy and judgement in the past, and had attacked China >3 times in the past to thwart China's modernization efforts. It had won every single time, except for the last time when it was forced to surrender to the Americans, but not to the Chinese, whom most Japanese despise. What's stopping Japan to gamble again, once it is nuclear capable??
Xi will be judged harshly if he allows Japan, an archnemesis of China, to acquire nuclear weapons under his watch.
5500 is not necessary a fact, there is a lot things needed to get done to get nukes and the mass producing them, remains to be seen if the material is really nuclear weapons grade or require more processing, they will have to test a device even if the US given them a reference design which the US is not fool enough to do that. Also China has and is capable of producing Tritium that allows them to build city buster megaton range weapons is not know if they that capability.
The thing with nukes is that once you have nuclear weapons you nation become target of nuclear weapons, even if your nation is not involved in a nuclear war, for example if the Russians and US are in nuclear war and Japan has nukes is highly probably that the Russian will try to destroy that arsenal just by the fact the Japan and the US are allies. Either way, China are not fools, they know that the US will try in a conflict to take their nukes before they can use them,
China has make crystal clear in their policies that any use of nuclear weapons in China will result in a huge escalation and in a
massive retaliatory attack against US cities, and China is not playing, they are going for the kill,
they are going for mass casualties given their arsenal, the are going to hit every single big US city. So that leave many US politicians with a
pretty big dilemma if things go south in the strait
are they willing to sacrifice Los Angeles for Taipei?
In the hypothetical scenario of nuclear proliferation in East Asia that dilemma doesn't disappear, in a hypothetical scenario politicians for example will have to decide if they are willing to sacrifice a city like Tokyo for example and 5,000,000 people who live there for Taipei? is a dilemma. China may have decided that their territorial integrity is worth a nuclear exchange if necessary. I don't know if is the same for other nations instigating a conflict like this hypothetical scenario.