—”but I am sure they have stockpiled all the components, carefully kept separate, and have run all the supercomputer simulations they need to make sure that, if they ever needed to be snapped together, they would definitely work.”
Far more technology is required to produce a Toyota than a basic nuclear bomb.
Has Japan started its new nuclear fuel reprocessing plant last promised for this year? Plutonium by the ton.
Japan already has more than enough nuke-quality plutonium on hand to build over 300 nuclear bombs, far more than China has. Japan had so much plutonium they sent the excess to the USA when the USA requested it. They don't need to create more plutonium, unless they want to give it to Taiwan or South Korea.
According to some articles, Japan has enough plutonium to make 6,000 nuclear bombs. But that plutonium requires some finessing to do so. On hand, they can rapidly assemble 300 nukes, within a few days.