One of Stalin's prequisites for allowing Kim start the Korean War was that Mao, and not him, would be the one watching over Kim's back. Stalin was not willing to have direct war over Korea and had been prepared to see Americans conquer all of Korea if China was unable to fight the Americans back.
I doubt there would have been escalation in Europe, but Soviets would likely have responded in US starting to bomb Chinese targets by further expanding their role in the air war. Chinese and Korean pilots had poor performance against US pilots, but the Soviet pilots flying newest aircraft had performance level comparable to US forces. Simply expanding the air war over China wasn't a magic solution that would have allowed US to achieve total victory.
I doubt there would have been escalation in Europe, but Soviets would likely have responded in US starting to bomb Chinese targets by further expanding their role in the air war. Chinese and Korean pilots had poor performance against US pilots, but the Soviet pilots flying newest aircraft had performance level comparable to US forces. Simply expanding the air war over China wasn't a magic solution that would have allowed US to achieve total victory.