If the West does not send NATO units on a massive scale, the Russians would eventually win this thing. There is no need to use nukes even inside Ukraine. The accuracy of today's weapons has improved so much that for small unit engagement, it is better and cheaper to use non-nuclear means. All the various aid packages from the West prolongs the war, but ultimately it comes down to firepower and armor. The West must send sufficient quantities of it to make a difference. If we send them piecemeal to Ukraine, without air superiority, they will be chewed up and spit out. few tens of thousands of Polish troops and Western "volunteers" will just delay the inevitable.
If the U.S. starts to amass large number of armor divisions along the Ukrainian border, I would be concern of tactical nuclear usage by the Russians. In a conventional war, the Russians will not be able to take on the U.S. led NATO troops and will resort to tactical nuclear to stop these troops. After that it is up to the U.S. to escalate to a full on nuclear war. I am seeing some mission creep like in Vietnam war, but hopefully cooler heads prevail and the U. S. will not pick a direct fight with the Russians in Ukraine.
In this war, China is like the U.S. in WWII, Russia is like Germany and Ukraine and the West is like Russia in WWII. In this case, the Chinese stays neutral but aids Russia in non-military ways. If NATO pick a direct fight with Russia, the Chinese will provide direct military aid to Russia and with the huge Chinese manufacturing base and the various military products, there will be just enough military aid to grind down NATO in a very long stalemate. At the end of this, Europe will be in shambles and the U.S. will be eclipsed by the Chinese without a fight.