People here use to overestimate nuclear weapons, especially earlier ones so there is a great possibility that Stalin and his government could have been largely survive it.
Not to mention we aren’t talking about particularly easy target.
In 1941 Germans abandoned idea of air raids on Moscow very early despite the fact that the Kremlin was within the range of their shortest ranged aircraft.
In a first raid alone they had only slightly over half of the bombers making it further suburbs and it become worse later.
The most valuable target they have managed to hit was a British embassy.
Not to mention a strike on Kremlin wouldn’t have made any different effect on Communists than nuking Mecca which is a daydream of some local armchair strategists on Islamists.
An idea to defeat Soviets in Europe conventionally is laughable to say the least too.
And the most important, how do you think the US government could sell this new war to a public?
IMO, Churchill was fully aware of the above and US was a target in his plan on par with USSR.
He wanted to tie both countries in conflict and try to maintain his empire.
If he really cared about an iron curtain he had to voice his concerns earlier in the war in Yalta and especially in Tehran when he still had leverage over Stalin.
If the Nazis took out Moscow, that could have very well been the end of Stalin. He certainly had enough enemies in the Red Army, and some optimistic Politburo member could have made their move.