About the possibility of using nuclear weapons, I'll recommend this article from 1990:
LEAD: The most pressing nuclear proliferation problem of the 1990's may not be, as is widely assumed, that more states will acquire nuclear weapons. It may turn out that nuclear weapons will acquire more states.
www.nytimes.com
Or step up to main conclussions:
[Speaking about China...] The risk of nuclear weapons becoming involved in internal conflict is even greater in the Soviet Union. There are nuclear weapons in most of the 15 republics; each tank rifle division has a nuclear-rocket battalion. In contrast to China, where virtually all soldiers in nuclear units are Han, there are significant numbers of non-Russians in Soviet nuclear units...
The risk of Putin using nuclear devices is high but I don't see many scenarios were this could benefit RU Fed at any term while US wants to keep supporting UKR.
What stopped the complete fall of "Russia" in the nineties could be also, just guessing, the risk of a civil war so the posibility of a nuclear catastrophe "indoors" and the repercusions on the rest of the world ended not involving West on that posibility, ending just with a supervision of the nuclear arsenal and allowing Gorvachev to handle the situation.
That is the real pandora's box Putin, in my opinion, is using against West today. "I know the war is lost for me, but the things in Russia are not going to be good for you as far this war keeps going with poor results for me"
Calling to a partial/general mobilization would greatly increase the chances of a civil war in a Russia where after decades of selling tons of materials to the rest of the world, its
prosperity score remains on 70th global ranking position, once they were the second largest global economy now mixed with some basic sparks for any "revolution": Corruption at high levels, unequality and discretionary killings of their own people.
-So using a nuclear device would put even some allies against them, an agreedment between China/Asia, US and Europe that benefits them also in economics is a real posibility as we know what benefits Chinesse economy, they probably have to stop their plans for Taiwan for more years, but instead they can keep giving high GDP growings to their ppl and come back stronger in years to come.
-Not using a nuclear device, losing the war and let falling the cards to the point of a civil war. This is a a scenario with a huge amount of variables, as the NYT article states, almost all fiveteen republics have nuclear capabilities.
One of them could even be, main imperialists remain in power in Moscow and a republic threatens them on accept its new sovereignity or instead of, Moscow will be destroyed, so Moscow could even threaten West to stop the revel republic with the condition of nuke main West cities if an "internal" nuclear weapon is used against them.
We are treating Moscow and Putin as one entity right now but bear in mind Moscow is the center of everything in Russia and they don't have the natural resources the rest of the Federation has. The willing of getting rid of corruption (mainly sourced on those close to stablished political system), the wish of advancing on equality and prosperity could create also a huge amount of variables and probabilities.
IMO, West has to have some plans for this scenarios right now as the war keeps on.