So we settle the 'Manchurian Question' by labeling it 'Imperial China'. Fair enough.
In regards to Korea, didn't the Two Koreas unify during the Twilight period? I don't have the books directly handy but I recall reading that. I would imagine that such a state would be controlled by the traditional capitol of Seoul but that Korean society would become more authoritarian and 'right-wing' with absorbing so many PRK citizens. Not quite Socialist, but definitely more conforming. Fewer professional gamers and K-Pop otakus and more focus on work, education and other 'approved outlets'.
And now that I bring it up, a constant concern of Korea 2300AD will be the re-militarization of Japan. South Koreans dislike the PRC, but they reserve a very special hatred for the Japanese. The only thing that keeps that in check currently is the PRC's open sponsorship of the PRK. There's a lot of unaddressed bad blood from the multiple medieval invasions from the Japanese and the occupation of Korea between Wars One and Two.