It depends really. If Moscow/the CNS can get its act together and prove themselves, then giving Siberia back would go a long way to helping Russia/stabilized it. (But with lots of business interests that goes in favor of the USA and China.) If they try that, and the CNS are doing a good job, I can't say the Russians in Vladivostok and so on would enjoy being more, or less a joint American/Chinese puppet state.
If not....your Independent Siberia still works, but only now it is because Russia simply can't get its act together and not because it is a insane 1984 style nation.
As you said, Siberia doesn't need a Neo-Stalinist regime to be plausible. Just having the CNS go down the road of the provisional government, of a nation of well-meaning but corrupt or incompetent officials, would be enough to have the Siberians be given independence. Having a Putin-esque strongman in Moscow definitely wouldn't make the Allies inclined to give the resources of Siberia to them.