What's the latest POD possible needed to create an Ainu-dominated Japan by 1900, that's a major world player by the mid 20th century, and creates the fewest butterflies possible for the western world, for as long as possible.
Best case scenario - Ainu dominated Japanese home islands
Medium - Independant or western-controlled (or Chinese) Ainu Hokkaido.
Unacceptable - OTL
Sorta inspired by this
thread.
Basically I don't think its possible without a POD in prehistory. The problems the Ainu had were 1) low population density which was basically flooded by immigrants from mainland Asia; 2) they were still at a Stone Age tech level when the Asian immigrants arrived, while the Asians were using iron and bronze when they arrived, evidently sometime between 1000 BC and 300 BC. If you allow the Asian migration to occur as per OTL and the Asians to chase the Ainu out of most of the Japanese Archipelago, the Ainu are pretty much done for.
In order to allow the Ainu to remain in control, at least two major changes have to occur.
1) Rice or millet farming needs to spread from Asia to Japan at a very early date...or, alternatively, there needs to be some sort of high-yield grain crop which is native to Japan, and which the Ainu domesticate at an early date. Either of these will allow an expansion of Ainu populations and the formation of organized Ainu states, capable of resisting the Asian migration.
2) The Ainu need to discover bronze and ironworking at an early date.
Another good thing, although not absolutely necessary if the first two conditions are met, might be for the Ainu to develop much more as a maritime people, allowing them to build a navy capable of defending their shores.
Possibly the early development of a more maritime-oriented culture for the Ainu could assist in the accomplishment of the two absolutely necessary conditions. A grain crop and metallurgy could possibly be gained through early trade contact with China. Perhaps we could envision the Ainu as the Minoans of the Far East, making their living by trade, and in the process gaining the technologies and resource base they need to resist the Asian (Yayoi) migration when it finally comes.