Let's say that Macarther uses nukes explicitly against Truman's wishes what would be the result
MacArthur is fired on the spot, and there is likely no outpouring of support to reinstate him.
The wrong lessons are probably learned at this point--that nuclear weapons can be used in defense of a nations' sphere of influence. NATO is basically saying as much in Europe, now its a reality.
So we're looking at things like the Soviet Union considering nukes against Hungary in 1954.
The Cold War ends in 1962, after the nuclear destruction of Washington DC, followed by the destruction of the Soviet Union, most of its people, and a general devastation in Europe.