even without this scenario by the end of the War Japan had lost near their entire Navy and Air Force and was essentially out of oil and other war materials with their manufacturing capability essentially gone. So Island hopping is going to be kind of difficult. Also there's the whole thing of being blockaded by the largest and most relatively powerful Navy in the history of the world(US Navy was over 6000 commissioned ships strong by the end of WW2 and still had new ships being churned out at an astonishing speed). I mean even a relatively short "hop" like the Korean strait is still about 120 miles and would be heavily patrolled and guarded at sea, by air, and with coastal defenses. Pretty sure Wolverine has admitted he can still drown.