Japan will always turn isolationist. The only way Japan won't be isolationist is if they become Christian or if the Mongols conquer them. Further, the Shogunate will always rule Japan until they're inevitably defeated and the Emperor (either Kenmu or Meiji, no exceptions) restored. Restoring the Emperor always makes Japan stronger. Japan needs very special circumstances to want to expand into Hokkaido. It's just as likely the Russians will conquer the island and it will be Russian-speaking as it will be Japanese.
Korea remains unimportant to any TL set before the Russo-Japanese War. It's only fate is to be colonised by either Russia or Japan, assuming it does not manage to conquer Manchuria.
The only Southeast Asian country which can escape colonisation is Siam. Everything else will inevitably be colonised by Europeans. Burma, Vietnam, etc. have zero chance.
Viking and Chinese America because of Lief Ericson and Zheng He respectively
Very true. Vinland can't succeed without Leif, and Chinese America can't succeed without Zheng He being involved, although the latter isn't as common here I notice since it's become almost as much of a cliche as "CSA abolishes slavery by 1875 and becomes a superpower". You also never see in Chinese America scenarios the groundwork needed to colonise the West Coast, that is, all the island hopping you need to cross the Pacific (either the "short" route up the coast of Japan to Kamchatka and Alaska or the Manila galleon route).
Chinese America is by far the most likely East Asian America. Japan, Korea, etc. have no chance at it. Excuses will always exist for China wanting to engage in voyages and expeditions like that but never for Japan or Korea.