Oh in my mind the Japanese accept for buffer reasons, not profit reason. Better to subsidize a weak country than have the Soviets on your door step.
The Imperial Japan seemingly did not have any problems with the Soviet neighborhood because almost all the way to its declaration of a war in 1945 the Soviets had been an important trade partner supplying Japan with the raw materials. After Japan withdrew its troops from the Northern Sakhalin in 1925 it was granted oil and coal concessions on the Soviet part of the island and and in addition the right to exploit “mineral, timber and other natural resources in all parts of the territory of the U. S. S. R.” in an addition to the fishing rights along the Pacific coast. Japan actively supported the So during the conflict on the Chinese Eastern Railway in 1929 by refusing to transport Chinese troops on Manchurian railroads and declining to take part in representations by other powers. In 1935 the SU sold its interest in the South Manchurian Railway to Japan, etc. After the clashes of 1938 and 39 (which Molotov declared to be dictated “not in Tokyo but somewhere in Europe” a non-aggression pact was signed in 1941. In 1944 a new convention was signed: In return for transferring to Russia all her rights and equipment in north Sakhalin, Japan received a payment of five million rubles, a contract under which Russia is obligated to supply Japan with 50,000 metric tons of oil a year for five consecutive years “after cessation of the present war,” and was permitted to remove her stored stocks tax free. Previous to the present treaty, Japanese concessions were due to run until 1970.
OTOH, if in the early 1920s Japan, after it was forced by international pressure (from the US at the Washington Conference on the Limitation of Armaments) to abandon an agreement regarding a buffer Far East Republic, Japan sticks to supporting some “buffer state” on North Sakhalin, it may see not only the problems with its future trade relations with the SU but also a continued pressure from the US and probably Britain as well.
Even better for them if Green Ukraine is part of the deal.
Which is extremely unlikely (but interesting idea): in the area in question the Ukrainians amounted to 18% of a population of the Green Klyn. So why would the Japanese or the Soviets bother with creating a separate state for them on Sakhalin?
That area at least has some more profit and sustainability. It also gives them another foothold on the mainland and in to Manchuria proper assuming the Japanese stay on the same course they did OTL. It’s not a perfect idea but it’s the best I could come up with for someone willing to get involved.
There was a Far East Republic (a presumably non-communist buffer state) created in the region per Soviet-Japanese agreement. Cutting few small Ukrainian enclaves out of it would not suit any of the sides. Anyway, why would the Ukrainians be more “Western” than the Russians?
As for how you support a population there when it’s already pretty maxed out, I don’t know how. Maybe long term it becomes ethnically Ukrainian as Green Urkrainians migrate to the island after claiming it in regardless of ethnicity when they declared independence. That’s really the best I can come up with short of population transfers for no logical reason.
That’s the main problem with the OP: required logic, logistics and economy is hard (if not impossible) to come with.