So far there's been no indication that Russia is willing to concede Donbass, let alone Crimea, will ever be recognised as Ukraine again by them. So I don't think that's a likely scenario at all, unless Putin is suddenly out of power - but short of someone protecting him putting a bullet in his head, I don't think the likelihood of Putin not being in power is very high.
Ukraine's got no incentive to negotiate with a Russia unwilling to recognize all Ukranian territories as Ukrainian (including Crimea). Nor should the rest of the world encourage them to appease Russia in this way. We know appeasement of expansionist authoritarians does not dissuade them. I'm not even sure why Ukraine would trust any peace treaty signed by Russia, considering Russia tore up the 1994 agreement that gave USSR nuclear weapons stored in Ukraine back to Russia. And I'm sure everyone involved in that from Ukraine's perspective is really regretting that agreement now.
If Russia is desperate enough to use nuclear weapons, it means they're desperate and stupid enough to kill us all. Because we all know how the world will respond to those nuclear launches. But I don't think Russia will launch nuclear weapons for the same reason the US and Russia don't invade countries with nuclear weapons - it guarantees their own demise.