None of what you are describing makes sense.

If the Ukrainians can push Russians back why wouldn’t they do the same everywhere? Crossing the border becomes an act of aggression, only Russia has acted aggressively here. Everyone else is trying to avoid conflict.

If Russia detonates a nuke in Europe the entire country will be obliterated immediately.

Ukrainians already have, there was a counter attack in the north of Dombas

That was a tactical strike on an airfield. They don’t hold Russian territory. They have not secured a bargaining chip.