No country on earth would want a superpower next door (ofc), unless they are a puppet or there's absolutely no other way than to comply. That's just the way it is.The ideal solution would be to have political settlements on any outstanding issues with Japan and Korea. No country in the neighborhood would like to become a burning pit for US self-interests.
Also, no country on earth would also want to be a lure-bait and a playground for an another country without any interests, it just doesn't make sense.
BUT if a country decides that the emerging superpower next door is such a threat that it is worth taking immense risks no matter the costs, the second paragraph becomes reality.
In Korea's case, there still are possiblities. China still has a long way to pass the current hegemony in both economy and military, (with economy being the more important one). Remember, the future hasn't happened yet, and we can't know what the future will bring. Remember, no one would have thought that there would be a global pandemic just 5 year ago. And that "little" pandemic shook the civilisation and the global order. Meaning, the future has not been set yet and anything can happen,
Anyway, the point is, in S.Korea's case, neither the pros or the cons of containing China alongside others has outweighed the other yet. They are an important U.S. ally and they are disturbed by the growing superpower (as any country would be, that's just the geopolitics for you) BUT they are still cautious. That's why they're setting their own path, you can mostly see this by looking at their defence industry. They haven't decided which way they want to go yet, they are choosing neither side and instead make their own way, or at least for now.
But in Japan's case, they are following the U.S.'s lead, but the uncertainty is, in the event of a crisis, will they help the U.S.? It's impossible to guess but the possiblities are 60% to 40%, imo.
Anyway, I'm avoiding to write more since I know that I can write a page of this topic but in short, these are my guesses.