Just saw an interesting analysis (unfortunately did not catch the fellows name, though I have seen him before) on why NATO involvement at this stage would not be wise. It is not one I had heard articulated previously, but it struck me as very insightful. He suggested that NATO involvement would not save Ukraine, in fact, Ukraine would become a bigger battlefield with even more destruction, with greater casualties, but further to that, it would give Putin the enemy he so desperately wants and needs. Currently, the Russian people are starting to rise up in protest, there is not great support at home for this war, Ukraine is not seen as a threat or enemy; NATO involvement would change that, Putin would be proven correct, ie, Ukraine is a NATO puppet and a threat to Russia; the Russian people would then rally around Putin and his war.
Unfortunately, I found this very a plausible and persuasive argument.
I again think we need incentives to get Putin to reverse course. I am in no way suggesting he get a free ride, I suspect whether he continues or pulls out he is not long for his current position. It is not going well for him. Don't give the Russian people a reason to rally to his side, the more they hate him, the better.