I really do not understand you.
"Attack by China" is full-scale war. Even in 10x10 square kilometers. First action of any counrty - to declare war and send regular army. If conflict continues or huge masses of Chinese forces arrive - nuclear strike imminent. There will be no "oh, sorry, we will give you siberia if you move out your forces from Moscow". This will be Third War. Maybe Last by coincidence.
This is not the way that wars between large countries have been being conducted in recent history.

It is called a "proxy" war. These sorts of war have been fought frequently throughout recent world history. Another name is: "low intensity conflict".

An attack by China does not necessarily equal a full scale war. A full scale war is a full scale war.

There will be no "oh, sorry, we will give you siberia if you move out your forces from Moscow".
I am not suggesting that China will move its forces into Moscow. I am suggesting that they could initiate and support an insurgency.

I am not suggesting that Russia will offer to give up Siberia if China withdraws. I am suggesting that an insurgency could take over one or two Chinese dominated cities in Siberia in the distant future.

What you are saying is simply wrong. The United States has backed Chechen rebels in the past and Russia did not respond with nuclear weapons. What I am saying has already happened in Russia.

You should give the Russian government a little bit more credit. An insurgency that was backed by a major power was suppressed without the use of nuclear weapons.

Unless you are saying the US did not support the Chechens? I a open to that possibility. Their is only anecdotes about it that I am aware of (the former FBI director said the US supported the Chechen rebels).