I don't agree with the premise of that hypothetical. Nuclear blackmail is only an effective strategy if it seems credible i.e. if the blackmailer is really in a desperate situation. If putin is faced with a situation where he would lose control if he doesn't use nukes (which we are not anywhere close to being) then by definition he probably doesn't want to be in a situation where blackmail would be effective.

There just isn't a lot of incentive for an autocratic ruler to destabilize his regime and crash his country's economy to grab a piece of territory at a massive price that the world will never recognize. Putin wanted to annex six eastern provinces of ukraine and create a client state in central Ukraine; not be in a position where he needs to threaten to lob nuclear weapons. Putin was able to grab Crimea precisely because there was no military resistance not because he was able to create a fait accompli that he was able to blackmail the world into accepting. Yet the sanctions imposed after Crimea was annexed dramatically weakened the civilian economy and putin's military buildup.

To be clear, I don't think Ukraine should negotiate or be forced to negotiate but that isn't because I believe Putin would not be deterred in the future.