What If Russia Uses Nuclear Weapons in Ukraine?

I do not agree with this. While there is not much actual tactical advantage for Russia then they are not distinguishable by very sane judgements. Any same person would have concluded that attacking Ukraine would inflict huge losses, yet this decision was made. This means that ruling out Russia using nuclear weapon against Ukraine is not possible based on this principle.

An air burst don't produce much fallout. And the really dangerous fallout from a ground burst don't reach that far, and it lasts only a few hours.

Russia may suffer some fallout, maybe enough to slightly increase cancer rates in some areas, but I guess that after using nuclear weapons, Russia will have more pressing problems than a couple of people getting sick.

A false flag from who?, No one involved in Ukraine has nukes but Russia.