I think the most likely scenario is that the war will fizzle out much like how the covid hoax did. The media will pivot to some other crisis like nothing happened.
However if escalation does occur, the best case scenario is as you described, a joint first strike with China crippling the US military so much that they are forced on the back foot and to meekly impose useless sanctions. The USA is in a similar position to Britain before the Suez crisis; not able to exert the geopolitical influence it once did, no longer being able to whip the global community into line and impose its will militarily. I think for all the talk about Russia being incompetent, it is actually the USA military that will be badly shown up as incompetent should war happen, and the American public will have no stomach for the thousands of dead soldiers that will result, unlike Russia and China which will view a world war as an existential crisis.
If America strikes first, hypothetically moving troops into Ukraine, putting a carrier group in the med and baltic seas etc, then I think it would actually be the worst scenario for them and this is why they are desperate for Russia to escalate first. It would unite the global community against America, it would kill the petrodollar, it would spell the end of America as a superpower. More practically I doubt they would accomplish much militarily. Can you imagine the demoralisation of America losing carriers, jet fighters, spec forices, all televised, while being unable to push Russia out of Ukraine since Russia can much more easily mobilise troops? America could probably not last a month in such a climate. It would be much worse than Vietnam or Afghanistan.
(post is archived)