I think the key to weakining Russia in an acceptable manner would be to shave off its western SSRs only, and integrating them somehow within the new European framework. They have the most nationalism at this point and the best historical precedence. The Baltic countries have been independent in the interwar era, Belorussia and Ukraine can call on common history with Poland, and together they will take ~30-40% of the Russian industry and population with them.
For the rest, I think the key to keeping Russia harmless will be in small cessations of territory as well as regulations of Russian internal policy.
If China takes back the Russian far east (Transamur), that will be a relatively small territorial cession, but one which will in one strike end any and all pretensions of Russia being a Pacific power, or ever threatening Manchuria again. With Vladivostok gone, their only other Pacific ports are inhospitable places like Okhotsk or Magadan. They suffice as small ports from which to maintain communications with Kamchatka or a small fishing fleet, but nothing else.
The key to keeping the remainder of China's new Siberian frontier safe would be to impose a federal structure on Russia's transural possessions. If the provinces can make their own economic agreements with China, without consulting Moscow first, that should link them closely to China and could create very good synergy. Chinese help in the exploitation of Siberia's natural riches would strengthen the region as a whole.
In Central Asia I think the situation will be uncritical for China as well. There need not be any independence of the 'stans, after all the Russo-Chinese frontier is one of the most inhospitable mountain ranges and strategically very safe for China. China's strategic interests beyond its borders woud, here too, be best served if the Tashkent/Ferghana region within Russia was turned into a federal subdivision of Russia, with the authority to determine its own economic policy and make economic agreements with China. The Soviet administration of the region was 100% tailored to doing the exact opposite - balkanizing the region as much as possible, stifling any hints at local independence, and exploiting it to an unsustainable maximum for the benefit of the remainder of the USSR. China would be well advised to undo that, and set up a local power center within the Russian federation, a power center that can assert its own interests against Moscow. I'm thinking along the lines of uniting the whole Almaata-Tashkent-Fergana-Samarkand region into one federal entity within Russia, with an elected Duma of its own, its own economic planning agency and a big credit line with the Chinese Central Asia Bank to fuel its economic development.
