The isolationism in America prior to Pearl Harbor was also related to the high level of German and Italians then living here not to mention many Irish with a grievance against Great Britain rather than a effect of communist influence among the general population of America.
American isolationism was a product of the ghastly carnage of the trenches in World War I and the evident failure of the Treaty of Versailles and the pathetic league of Nations to curb what Americans regarded to be a penchant for war among European nations.
The general mood for noninvolvement was largely unaffected by Hitler's invasion of Russia, but you are quite correct the previous neutral posture of the Communists flipped entirely with the invasion of Russia. The general population quite naturally sprang almost unanimously to favor war simply because of the Pearl Harbor attack. Hitler obligingly relieved Roosevelt of the problem of waging war against Nazi Germany by declaring war on America and even made it possible for the allies to prioritize the war in Europe over the war in the Pacific with that act of folly.
I would agree that the imposition of the oil embargo made Japan's decision to wage undeclared war more likely but not necessarily inevitable. The oil embargo was entirely justified in view of Japan's brutality in China and aggression in Manchuria. Japan's decision to wage war was the product of a military class which simply would not consider any other option but might have acted otherwise if the Emperor had weighed in against the attack on Pearl Harbor.
