oh I know what you were referring to.
I like to reference a move quote here.
"you really should be glad about this carl, me working for you, you end up being my bitch."
I'm fond of Japan, nothing personal, just the truth.
In your scenario, basically it's game over for Korea and Japan, as separate ethnic with their own living language and culture (especially Japan royalty). Also, I doubt the Chinese will accept Japan and Korea as their President/PM.
Just look what happens to Manchu or some of the Chinese local dialect. Even if China Government has no ill will, just by their sheer population number, sooner or later mandarin will replace both languages. Similar to what happen in Taiwan, in Taipei, it's getting harder to hear someone speak Hakka in the street.
I do not think it would be "game over" for Japan and Korea, rather I believe it would be a beginning of a new Regional Identity. Where once it was based on national lines, and political rivalry, the region would be devoid of such political instability.
The Manchu as their Jurchen cousins were never numerous . Japan is not small by any sense as we are over 128 million, and Korea is about 80 million. 200 million or so combines. So there would be a healthy combination.
Lastly, about the Imperial House. The Imperial House does not necessarily be a limiting factor as it merely will remain a ceremonial institution in Japan.
Why we want to learn Nihonggi abd Hanggul when Mandarin is already a standard dialect than unify all dialects (Cantonese, Hokkien, Tewchiew, Hakka, Hainam...and many other chinese dialects) in China, if Japaneses are sincere to have an integrated Asia, you should as adopt also Mandarin as your official international communication dialect as the ancient time when you your ancestors used Kanji to communicate with Koreans & Chinese people, Chinese characters was the official communication tool in ancient East Asia that include China, Korea, Japan and Vietnam...and it should stay the way it is.
And you think Japan's emperor is Japanese?
Japan guards the emperors' secrets - World - News - The Independent
There is no denying that there is Korean blood in the Imperial Family. We are all interlinked somehow.