Japanese Sakhalin and the Kuril Islands

What if the Japanese had settled in Sakhalin and the Kuril Islands before the Russians had, and they became part of the Japanese home islands?
 
How about an earlier contact with the Chinese and Japanese monks (as well as officials) in both Sakhalin and Hokkaido, but I admit that's so ASB.
 
What if the Japanese had settled in Sakhalin and the Kuril Islands before the Russians had, and they became part of the Japanese home islands?

This seems immensely difficult to do. Hokkaido was the periphery of Japanese civilization for most of history. Going beyond that just is not likely before the 1500s, and once Japan closes itself off, expansion is impossible. So, either Japan does not go isolationist, or it departs isolationism sooner.
 
If you're going to ask after a divergence like this, you're meant to call them Karafuto and the Chishima archipelago, as Japan did.
:p

More seriously though, I'm not actually sure how you'd accomplish this.
 
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