PC: What's the best time for a Russian takeover of Korea?
Besides Japan, Russia also had a chance at seizing Korea. Its position in NE Asia is extremely vital, and a Russian Korea would have allowed the Russians to acquire multiple warm water ports. However, Korea in the hands of a third party power would also be a threat to Japanese national security, which is exactly why the Russo-Japanese War happened, with Japan ultimately emerging the victor and annexing Korea five years after the RJW.
However, if an earlier Russian push towards the Pacific occurs, they could have also been in a position to conquer Korea as well, with an earlier establishment of a Russo-Chinese border at the Amur River way earlier.
If Russia ends up with Korea, then Japan could at least be compensated with being allowed to take the Philippines from Spain. Unlike Korea, a Japanese Philippines could secure more sea lanes for commercial trade, and also safeguard Japanese Taiwan as well.