If China has to nuke Russia in order to restore Chinese rule over Chinese territory then they will.
Better Russia should give these lands back to China instead of risking WW3 over them! Or maybe they could negotiate over how much land China will take for now. Oh, and I imagine China will want to denazify these areas. That’d be sensible.
The Russians are spread pretty thin on the ground compared to the Chinese so the Chinese better be willing to lose 500 million to maybe 40 million Russians.