The Chinese will be deeply hostile because they see the newcomers as rebels even if they don't seek to control the north. Japan ironically might seek to be friendly but their probably the power the UTers will trust least. Britain might be an option as its interested in trade and also checking Russian expansion while there's no alliance with Japan yet but its also got a hell of a lot of trade and influence in China so that could be a significant barrier. France as a republic might be friendly but has relatively little influence that far north. The best bet might be the US, especially as that's a power the UTers might trust but it also has little influence in or trade in the region, isn't in its external expansion phase - outside the continental US quite yet and also has its own dislike of east Asians. Russia is possibly an outside bet, especially since imperial Russia might find some of the information very interesting [and worrying] but would a reactionary Czardom be that good an ally and such a link would alienate China, Japan and Britain.
Here is some background from wiki on Korea's foreign relations status as of circa 1890:
From wiki:
Some contradictory info on the Chinese role here, first saying they permitted, even encouraged, Korean embassies with the west, then saying they sought to limit them. And ironically the "strategy for Korea" paper which recommended relations with Japan and western countries, was written by a *Chinese* diplomat in residence in Tokyo and handed over to Korean emissaries there circa 1880, and it was intended as a counter *Russian* threat strategy at that moment.
My thoughts on South Korea's situation?
China will be miffed and demand "explanations" about the disappearance of Yuan Shikai and its Chinese advisory group and be disturbed by reports of movements by troops with advanced guns, automotive vehicles, advanced coastal ships, and allegedly "flying machines" from southern to northern Korea that they will scarcely believe could be Korean rather than Japanese or western, or truly exist at all.
The Japanese will also demand explanations about loss of contact with their envoys and businesspeople in southern Korea, and reports they hear from Japanese in northern Korea, as ROK forces advance into the land.
I imagine the Syngman Rhee government will stiff-arm the Japanese and keep them at arms-length, not giving them many details, explaining the situation has supernaturally changed, explaining there are no Japanese residents in southern Korea, and assuring safety of Japanese subjects in northern Korea, and offering to provide safe passage home if desired to Japanese subjects present in northern Korea. The velvet glove would be combined with the iron first, of ROK gunmen at ports, patrols of some modern-ish steel patrol boats and minor naval combatants on hand to warn any Japanese fleet forces approaching to back off of any approaches or attempted landing in any part of the Korean peninsula, with willingness to fire for effect if Korean warnings are ignored.
The ROKs will probably show a bit more patience with inquiring and approaching western naval flotillas. In all cases, they will reinforce their own messengers, knowledgeable in foreign languages of Japan and the west, by using, by getting corroboration of third parties, mainly western diplomats from the diplomatic quarters of Seoul, like Americans, Brits, Frenchmen, Scandinavians, and whoever else can be found, maybe some Turks, Arabs, Indians, Australians or Latin Americans.
With the Chinese, the Koreans I think would want to be more generous and open with information. President Rhee's bottom-line is that the old Korean monarchy and class system is dead and is not coming back, the Republican Korean regime under his Presidency under him is here to stay, to rule the whole peninsula. But he is happy to have privileged relations with China and regard China as Korea's best friend and senior ally going forward. And he is happy to share views of the present, and news from the future, to show why this is best for both Korea and China.
He would take downtime Chinese, if they're willing, on a tour of 1950 Inchon's Chinatown, to show how it had grown from a community of a a couple hundred to a few thousand by 1950. Then he would also share the sordid history of the next fifty-five years of Japanese imperialism, aggression and occupation, against both Korea and China, to demonstrate he is no modernizing tool of Japan. He will warn what the Japanese have in store for China as soon as 1894, encourage them to let Korea be a strong buffer protecting northern China, encourage them to look under the hood to make sure their navy is working and can protect Taiwan and the Pescadores, and warn of the perils of the Boxers, and Boxer Suppression expedition.
If the Chinese insist on being bossy, making rules, taking over Korean administration of the south, trying to obstruct ROK assumption of control of the north by force, the ROKs will use decisive force against them, at first in demonstrative "lessons" designed to be followed up by asking "had enough?", but ready to drive out the Chinese garrison/intervening troops if their commanders/diplomats/central government don't get the message, and the Koreans just have to use their military tech and training edge while they still have it.
The Chinese of this era did not seem to have the virtue of military tenacity in the face of military failure, military surprises, or the unexpected, so despite their greatly superior numbers over the South Koreans, who don't have the industrial base to resupply ammo, spares and fuel for vehicles for months on end, and certainly not years, I just do not expect the Chinese to keep sending human wave after human wave against the Koreans.
Whether forced to demonstrate military might or not, and in small, low-key ways, or in larger dramatic ways that more visibly humiliate either China or Japan.....I doubt Britain, France, the USA, Russia, or Germany would rush in fast to try an occupation just for the fun of it. The ROKs, aided by uptime, mainly western, diplomats, and probably uptime ROC diplomats, would probably start trying to work diplomatic channels with Russia, China, the western countries, and with them as intermediaries, the Japanese, to get things to calm down before getting in over their heads or in too much trouble.
As I noted before, the Koreans will market themselves to Western Europeans and Americans as receptive to Christianity, especially Protestantism, which the Salisbury Ministry (I think) and Cleveland Administration, Dutch, Scandinavians, and Germans will like. And to Catholicism, which the non-secular French, Italians, Spanish, Portuguese, Belgians, and Austro-Hungarians will like.
Uptime ROC/Taiwan diplomats may have some distaste/disgust for the downtime Manchu/Qing dynasty, but they may be up for working with Han Chinese interlocutors working for the dynasty in order to support larger Chinese interests and to thwart Japanese or Russian or other foreign interests that could harm China.
The largest and dominating diplomatic and advisory presence in Seoul would be the uptime American Embassy and country team led by Ambassador John Muccio, and the Korean Military Advisory Group (KMAG). It would constantly caution Rhee and the ROK leadership about the extremely finite reserves of ammo, fuel and spares in country for all parts of the ROKs modern military. They would want to make contact with the American Ministers and Consuls in Peking and Tokyo and the Cleveland Administration in DC. The American, British, French, Canadian, Australian Embassies will all be of largely like-mind, and prone to want to provide their home countries "news from the future" - Korea should be an ally; watch out for Japan, it becomes dangerous, watch out for Germany, all three of us will need to fight it, twice, Russia will be unreliable and dangerous - we fight alongside it twice, but it undergoes a crazy, contagious revolution and becomes our most threatening enemy and subverts all China, half of Europe and half go Korea by sixty years from the present.
This will all give downtime Washington, London, and Paris plenty....too much... to take in.
In the meantime, Germany, Austria, Italy, Japan, and Russia will not have their own uptime embassies or diplomats in Seoul to provide "news from the future" back home to them, although there may be some private individuals who could do it. Possibly some southern Korean Communists who can score travel documents and can speak foreign languages could exit the ROK and communicate news from the future with members of the Socialist International abroad.
However, Rhee's ROK government could hedge its bets by reaching out directly to more countries than just the western big three. South Korea 1950 does not lack for fluent Japanese speakers, certainly. His own wife is Austrian and so speaks German, possibly Italian as well: Mrs. Syngman Rhee: Francesca Donner - Wikipedia
Austria-Hungary and Germany are distantly safe potential industrial trade partners with fairly weak navies and Far East armed presences at this point. So is Italy. The Russian Empire already has Korean speakers who can be interlocutors, and likely there are 1950 South Koreans who learned Russian as part of IJA or IJN service or working for nascent ROK intel....and they can warn the Tsar to take the Red Menace (and Japan) even more seriously.
Interestingly, any black Americans in military or diplomatic service in South Korea at this moment from 1950 to 1890 would not find homeland America as bad or as different from 1950 America as they would if they had been time-traveled to 1900 America, 1910 America, 1920 America, 1930 America or even 1940 America, but they would probably have an idea that things were not going to get better and in fact would be getting worse. They may well prefer to stay in service abroad and avoiding postings outside the southern states if they can do anything about it.
