The Ukrainians will increasingly be in the mode of having to hold territory while Russia would probably be willing to call it a victory if they level the place

The Ukrainians will increasingly be in the mode of having to hold territory while Russia would probably be willing to call it a victory if they level the place and make it uninhabitable for anyone, so I wouldn't assume they'll even bother aiming much. And let's say we do strike back with cruise missiles - it's unlikely to trigger Putin being overthrown. In fact, it may even convince some people on the fence that it really has been a war with the US for the survival of Russia all along. He has to retaliate so as not to appear weak, how can he reach an American target, even just an overseas base? I'm honestly not sure. The scenario where his nuke draws one in return is just as messy with the out that it's so utterly shocking that it forces both sides to confront the fact that no one wants a nuclear war. I really think China holds all the cards here - if they seriously threatened to cut Russia off over nukes, I don't think any get dropped. If they make wishy washy statements, there's a real chance they do.