"Yes. Yes I do. That's exactly what I think they will do. Because Russia knows exactly what will happen if they explode a nuke on Ukrainian soil. Moscow will shorty thereafter become a large glass bowl, or perhaps, a radioactive skating rink."

No. No it wouldn't.

North Korea is going to stick with Russia to the end. They are both Pariah states, so why not be Pariahs together. But if Russia were to use a nuke, that would likely be the end of the Sino-Soviet partnership, whatever that is. Ditto, the CSTO would fall apart completely. The seams are already present and growing, this would just be the end of the alliance. I think even stalwart allies such as Iran and Syria would face immense pressure to abandon their support for Russia.

And then there's the Russians. The vast majority of support for the SMO comes from older Russians who want the USSR back. That's who Putin is appealing to. Younger Russians are generally unhappy with the current situation, but they know that if they try and do something about it they're likely to get killed or imprisoned. A lot of those people who remain quiet may not if Putin brings on a risk of getting bombed by a coalition that may include countries from all over the globe.

"Hell. Poland could march on Moscow and take it in a month, as long as it wasn't winter..."

Change Poland to Finland, and winter is no longer a concern.

I wonder if there is any talk in Finland about launching their own SMO to retake Karelia and the Hand.