He who fires a Nuke has the certainty that he will be equally fired upon.
Only if you fire against someone who can fire back. Ukraine has no nukes, and doesn't have a mutual defense alliance with anyone with nukes. There is
zero chance that NATO would fire nukes against Russia in retaliation, just as they didn't send in ground troops in response to the ground invasion. (If Russia nuked eg. Poland,
then we'd have full-scale global nuclear war.)
On your other points, I hope you're right, but I'm worried that Putin
may actually be mad. I can't see how
any part of this Ukraine invasion is rational from
any perspective.
I agree that there is a probably-wrong perception that Ukraine is on a path to defeating Russia, though things
have clearly been going much worse for the Russians than they expected. It's hard not to focus on this little speck of justice in this tragic, unjust situation.
I think what might actually result from all this, is that Russia will be broken up into smaller countries, stripped of its nuclear weapons, and
heavy sanctions will be imposed on them to pay for all the damage they have caused. Think Germany after WWI.
Who's going break them up? Whoever controls Russia's nuclear weapons can stop any effort like that by threatening to nuke the aggressor, so you'd have to have the leader of Russia (Putin or his successors) voluntarily give up their nukes, etc.