I think the risks for Russia are just too big. Now UK and the US are openly supplying Ukraine and it's not going to be a surprise to anyone like Georgia was (if my memory serves me right). I'm no expert, and of course it's impossible to know what's going on behind the scenes, so ... you know. Still, wars are really really expensive, and it's a long commitment. Putin would have to be really cornered to start anything crazy.

Regarding China and Taiwan, I doubt China would risk an open war either for what would essentially only be national pride. Of course if there was a major war in Europe, in the scale were the US had to fully commit to, then maybe it would start to make sense. Still, expensive and risky as hell.