Why the Western Allies might accept Soviet landings in Japan

Would Britain and the United States have welcomed Soviet troops crossing into Japan?

Not strategically, but they could accept northern Soviet landings to reduce Western casualties and create additional fronts.

Long-term geopolitics and immediate battlefield needs point in different directions. Britain and the United States would probably dislike a Soviet crossing on the strategic level. Soviet troops arriving in northern Japan could still reduce Western manpower losses and force Japanese defenders to fight on several fronts. The Soviet Union had troops in Manchuria but needed ships, tying the political calculation directly to amphibious lift.