Not sure what the consequences of attacking NATO has to do with this.
He'd be backed into the door marked "exit". There is no corner to trap him here.
>End the invasion after suffering a nuclear strike
And why do you believe that Zelensky or whoever is in charge would nuke Moscow first? Do you think that, if they had say 30 nukes (plenty for a few relatively harmless demonstrations) that this would be the first target? Obviously they'd pick something that he could decide to de-escalate afterwards.
>they're only credible if used in response to a correspondingly severe threat.
You mean such as the severe threat that Ukraine has endured for a decade at this point? The war now threatens to make them functionally extinct. Many have fled and will never return, their population is reduced to something absurdly low, many of their children have been forcibly abducted to be indoctrinated or tormented/tortured.
That condition you impose was pre-satisfied.
>Certainly they're not an automatic "leave me alone" card.
Of course not. They'd have to be used intelligently (readers: "used" does not imply detonated). It's not entirely clear to me that this would be the case with Ukraine/Zelensky. But nothing less at this point will suffice. Even if the US promised to put 150,000 troops on the ground, this wouldn't end. It would only escalate. Perhaps to that nuclear war you seem to fear.
As has been illustrated so well over the past few years, the power of nuclear weapons is a paradox. It allows you to make the ultimate threat. But that threat isn't credible unless people believe you'll use them. Because the consequences of using them are so severe, they're only credible if used in response to a correspondingly severe threat. Russia's arsenal hasn't allowed it to stop a constant flow of weapons to its enemy, an enemy which has invaded and still controls a small bit of Russian territory, and which frequently carries out aerial attacks on Russian territory. Ukraine faces much more of an existential threat (Ukraine has no prospect of conquering Russia, but the reverse is a serious possibility) so a nuclear threat from Ukraine would be more credible, but it could easily still not be enough. Certainly they're not an automatic "leave me alone" card.