This is a challenge if you choose to accept it make Korea a Orthodox Christian nation in the POD of 1300 AD, who could likely do this to Korea, what would be the reaction of China and Japan?
This is easy. Russia expands more fervently into Chinese territory seizing Manchuria and the Mongol regions long before hand, as the POD has the unintended consequence of Muscovy becoming stronger in the wake of a faster Tartar collapse. With Poland providing a strong buffer from the west, Sweden from the North, and the Turks from the South that leaves East, and they march forth with gusto. Historically Korea was very open to the idea and the ideals of Christianity, it is not so difficult for Orthodoxy instead of Catholicism to take prominence. It just requires Russia expand faster or even just have Oriental Orthodoxy become prominent (though this is decidedly more difficult and likely involves Il Khanid-Yuan slave trade or soldier exchanges).This is a challenge if you choose to accept it make Korea a Orthodox Christian nation in the POD of 1300 AD, who could likely do this to Korea, what would be the reaction of China and Japan?
The POD makes this unlikely though without significant Mongol use.Russia expanding faster into Korea would have to occur sometime during either Ivan IV's reign, or the early stages of the Romanov Era. Oriental Orthodoxy on the other hand, I'm not sure if the Assyrian Church of the East would qualify. As I said before, Nestorian Christianity would be the only choice of Oriental Orthodoxy.