The Chinese had determined to commit troops if the “UN” forces crossed the 38th parallel. When they did it was not Macarthur’s decision. The US gov. (CIA) believed that the Chinese would stay out of Korea. They expected the Chinese to build a defensive line at the border.
The decision to use nukes was made by Truman. It was determined, when the front stabilized near the 38th Parallel, that if large numbers of Chinese or Russian troops were moved in to Korea, nukes would be used on specific targets in Manchuria.
Macarthur did very little out side of his orders. He was told to use only ROK units within a certain distance of the Chinese border. ROK units were of very low quality for the most part, so Mac used his judgment to the counter of his orders here.
Macarthur wanted to attack what he saw as a threat. That’s what Generals do. I don’t think the conflicts between him and the US Gov. make him any less of a General. Maybe if some of the German Generals were a little more like Mac and Patton they would have had a chance to show their true abilities in WWII.