Assuming that any nuclear weapons involved are only targeted at Ukraine I don’t believe that Nuclear Holocaust follows immediately. The United States has a direct line to the Kremlin for things like this and I’m sure that the other nuclear powers do as well.
In this “Russia nuking Ukraine” scenario I think Russia would have a bigger problem with the Europeans than the United States. The Europeans would be seriously pissed off and both the French and the Brits are well able to respond with a range of economic, political, conventional, and even nuclear options.
China would be hella displeased too, and not solely because of Taiwan, but because they would have to immediately choose between supporting Russia or doing business with literally anyone else in the world as the economic sanctions applied to Russia and any of their allies would be immediate and total.
Russia isn’t going to launch because the only possible outcomes is their total isolation from all other nations or their complete destruction in nuclear fire. Putin knows this.
Musk is only 53, I don’t think “geriatric” is a realistic description of him. Trump yes, but not Musk.