Ukraine couldn't have used the threat of nukes WRT Crimea. It happened too fast (IIRC Russians used old school messengers to avoid Western SIGINT), and at least the majority of the population supported or did not actively oppose them (e.g. Ukrainian generals defecting to the Russians)
It's another story WRT to Eastern Ukraine. Ukraine's military units had poor morale and were repeatedly destroyed by regular Russian soldiers AKA "volunteers". The time frame is not days as in Crimea. Perhaps the threat of nukes would have stopped Russian tanks and soldiers from "volunteering" in Ukraine.
It is now up to the armchair historians and alt-history writers to contemplate the what-ifs.
The utility of nuclear weapons is strategic, not tactical. The only relevant deterrent is their use against against major cities for which speed is not really relevant. What's relevant is maintaining launch capability in your control - hence major powers go for stealthy submarine platforms.
It's another story WRT to Eastern Ukraine. Ukraine's military units had poor morale and were repeatedly destroyed by regular Russian soldiers AKA "volunteers". The time frame is not days as in Crimea. Perhaps the threat of nukes would have stopped Russian tanks and soldiers from "volunteering" in Ukraine.
It is now up to the armchair historians and alt-history writers to contemplate the what-ifs.