And, asserting NATO air dominance in Ukraine is easy and guaranteed to succeed with de minimis cost (in lives and/or military equipment). Whereas creating land dominance with tanks is another matter.
A no-fly zone is an easy win for NATO and would make Ukrainian civilians safe from aerial bombardment.
NATO protecting Ukraine would not trigger anything.
I think this is a sensitive area where acting like there is any certainty is dangerous. I can't agree with your assumptions about 2nd order and 3rd order effects because there are too many known and unknown variables to consider.
This is not true. No nuclear powers should engage in direct wars, I'm not a fan of proxy wars either but they are safer than direct conflicts which can easily escalate.
We have never had a world war between nuclear armed states and we should avoid the potential at all costs.
Iraq has been invaded for a lot less; Kuwait population back then was 1/20 than current Ukraine, but Saddam never had nukes (as any other WMDs, but I digress). So what happens if say 3 years from now Russia decides to invade an EU or NATO country? Will it make any difference that the invaded country is part of the EU or NATO? They still have nukes, so what would be the western response? Will the nuclear deterrent magically disappear because of a NATO or EU stamp somewhere? That is not a trivial issue; ignoring the nuclear threat is for sure the safest way, but can create a nasty precedent in which every nation with nuclear weapons could be entitled to do what they want if they become rogue as Russia is today.
Doing this feeds the narrative from Putin that NATO is an aggressive force and it would also go against what NATO is, a defensive alliance.
"The teacher punishing the bully feeds the bully's narrative that school is unfair."
"Feeding a narrative" is never a reason to not do the right thing.