Comment about various issues:
@GrimmReaper: whether he was clueless or foolhardy is for the PoD's purposes irrelevant, it assumes he kept the insight and common sense he showed at Inchon and decides to build a defensive system at the 40th Parallel if the Chi Comm indeed show up. Since Truman greenlighted him to reach the Yalu, nobody will gainsay him for doing so. The big glaring American weakness of the whole Korea campaign was to strive for the Yalu in a totally unprotected way. If the Chi Comm indeed show up and MacArthur is able to ocintain them (as he will if he's ready to fall back on the 40th), the fault for expanding the war will fall squarely on the shoulders of Mao, in the eyes of the American and Western public.
I'm also persuaded that the Chi Comm show up in abundance but the US Army is able to cointain them, they will be righteously angered instead of disheartened about the whole enterprise, as a defeat would have them, so the President, the Congress, and the public will be more than happy to expand the US Armed Forces and the forces in Korea to deal with the Commies effectively. The 1950 USA are more than able to stop and smash everything Commie China can throw at them in the Korea theater without too much effort, if there is the political will. With a second victory at the 40th, there will be. Just expand the draft and the defense budget a bit. If Mao is blatantly throwing everything he has in Korea to defeat the USA, there is the justification.
@DuQuesne: The Soviet delegate is utterly irrelevant here. By late 1950, he had already came back, and the West was using the General Assembly to bypass Soviet veto. As strange as it may seem today, in those halcyion days, the majority of UN members were mostly pro-Western, instead of that loathsome ragtag of loonie anti-Western, anti-sionist dictatorships it became in the 1970s.
@LightInfa: this won't matter in the slightest. Provided the USA have a prepared good defensive position to fall back in at the 40th Parallel, and the necessary reinforcements and supplies are sent, they can absorb pretty much anything that Mao can throw at them, even if he bleeds the Chi Comm Army to do so. It will only throw that much more Chinese raw meat in the USA artillery-aviation meatgrinder.

If Mao is so stubborn at having a Communist buffer state beyond the Yalu as to throw any decent Chi Comm military unit at the 40th, all the good for the USA. The Chi Comm Army will be utterly bled white and become dangerously vulnerable at a counterattack from Taiwan.
Not to mention the fact that if Mao is blatantly throwing anything he has at the Yalu, the USA and the UN will most likely greenlight bombing Manchuria, even more of a "as many as you can kill" Commie-slayer party.
The one thing the USA ought avoid to do is to send USA ground units in mainland China. If it comes to that, bleed the Chi Comm at the Yalu and let the Chi Nat do the landings. However, I'm doubtful that the Chinese Civil War could be reopened for success, unless Mao bleeds the nation white so much for his Yalu adventure that he alienates the population against the Communists. But surely, if the 40th parallel option is used correctly, North Korea can be wiped out and a full victory achieved no matter what Mao does.