The common sense and what is written in the rules can be different, and there are many examples of this, not only in A&A, but in most of the late times games, board, PC or cards. A sad fact
Now, the rules simply say “at beginning of an allied power combat moves phase”. No mention on wich power or if the defender and who’s turn is must be the same. It says nothing, so Japan has full freedom. They could send even one kami at USA’s turn, another at China’s turn, and the same for UK and ANZAC turns, all in the same round
It’s not a teorical question, I have a mixed fleet in phi sz in a game and I want to know what to do. The important stuff is not China itself (Japan could kamikaze in UK’s turn that in fact is reduced in my game to Canada and nothing more, in fact is poorer than China)
It’s pretty possible we get a clarification anyway, because this seems another example of poorly tested rules