Kyiv falling is inevitable.
Killing Zelenskyy will be stupid because of the profile and aura that has been created around him. Rather leave him alive and ''humiliate'' him by getting your way through negotiations. Then Putin can go back home and sell whatever sh*t he wants to to his people. ''See, I'm strong man. I took Crimea; I took Kyiv; I liberated the Donbas and made the western puppet Zelenskyy concede to my demands''.
I don't think Putin would survive (as a leader) taking on NATO. The US and UK alone would be too much to handle militarily, now throw in essentially the entirety of the west (logistics, supplies, financing, etc) behind them and they won't survive a confrontation.
Nukes are off the table. If Putin uses nukes he condemns Russia in the process.