If the pincer fails, nuke would probably be on the table.
To even mention this would probably get in trouble, but there is a tiny outside chance that NATO will simply do what Germany and Russia did way back at the very beginning of WWII. Let Russia take a part and they also take a part - instead of trying to defend, simply occupy in the name of defense.
Splitting Ukraine between NATO and Russia would be a terrible outcome for most of rest of the world. Pretty much a direct copycat of what happened before.
Just a reminder that NATO is under no formal obligation to defend Ukraine. Nor would Ukraine have any defense against NATO should they decide to occupy. If the Ukrainian poplulation continues to mass exodus - NATO will occupy by default.