Is China being left behind in U.S.-N Korea sparring?

I think they're just sitting back, and eating popcorn while watching all of this.

No China wants to replace North Korea's idiot with China's idiot of choice and will let the US do it for them---To a limit.

China is doing nothing because it all benefits them no matter which way it rolls. Whether NK nukes the U.S., ore the U.S. folds and pulls it's troops out of SKorea, either way China will achieve it's goal of replacing the U./S. as the major strategic power in East Asia.