If outright victorious, then hopefully Putin is kept somewhat in check by China, as they are his only real lifeline at the moment. The CCP is very conscientious of its public image, and I do not think they'd want to be seen as complicit in Putin say lobbing nukes into Ukraine. The Europeans can look the other way on China regarding Hong Kong, Tibet, Uyghurs, etc...but will not if nukes are going off in their backyard. Also the United States as well can hopefully keep him in check to some degree. I think what would happen is he'd be forced to come to the negotiating table.
If a stalemate, well the West can likely support longer than Russia and the United States probably definitely so, but with the winter coming, that is going to complicate things some if Europeans are freezing. I am really wondering how it could become a stalemate though because the Russian forces seem to be losing so much that they cannot replace, whereas the Ukrainians are continually getting more stuff. It will take some time for them to push the Russians out because they have to build up the appropriate supply lines and plan a proper attack and all that, then after a successful attack, they can only advance so far before having to stop to rest, recover, and replenish and rebuild their supply lines again. But seeing how only seven months in, the Russian military already seems to be getting pushed onto its last legs, I am optimistic. Of course a great mistake in military history is to assume the enemy is a spent force and then you get a major counter-attack, but usually that is with a skilled force suffering resource losses, not an incompetent force like the Russians.
Would have to disagree there will be little gain for either side. It seems, thus far, that with continued proper support, the Ukrainians can have tremendous gain. I agree with you that a more radical leader could possibly replace Putin, and so maybe the West shouldn't punish too badly (i.e. like Germany after WWI), but allow them to integrate back into the world economy.