No, it's not working. There's indeed direct outside help. Ukraine doesn't need US troops, they have their own, they need weapons, training, intel, etc which is exactly what all the western nations have directly provided, and they've been very open about it.
There's also no need for a NFZ considering Russia doesn't have air superiority either... who are you going to prevent from flying?
North Korea is a nuclear state already. Much like Israel and Pakistan, they're not officially recognized by NPT, but they've nuclear weapons. And nuclear deterrence is very overrated: see Israel for a country with nuclear capabilities still being attacked.
There are no guarantees. Should Russia use a tactical nuke in Ukraine, I can see NATO just simply using conventional air superiority to decimate whatever small chunk of Ukrainian land Russia is still holding. Like I said, it can only get worse for Russia.