The Japanese and Koreans have been fighting each other since well before there was a North Korea. If the Koreans and Chinese are justified in mistrusting Japan, and they are, why would Japan trust them, given their history? Without the US, who has a bigger military, Japan or China?
You are alarmed that the Japanese possess plutonium but the Chinese have had actual nuclear weapons since the 60s. It is widely believed that the N. Koreans possess functioning nuclear missiles capable of hitting Japan. China is acting belligerently and imperialistically, they are claiming various islands and building military instillations on them. Are the Japanese doing that?