I think people are underestimating just how involved China is with North Korea and how little China wish for a unified Korea:
North Korea and China have a treaty according to which they are obliged to defend each other and provide assistance in the event of war. In other words, if the US bomb North Korea, China is required to come to north Korea's assistance militarily on the simplest interpretation. There are indications this treaty is not simply seen as a cold-war relic by China since the anniversary of the signing was celebrated last year.
North Korea's attempts to obtain ICBMs with nukes has been underway for decades and is now in the final stages. Obviously, China has long ago accepted this will happen, and recognize it as a far lesser evil than a unified Korea. Drawing red lines in the sand and hoping China will change its minds seems very far-fetched to me at this point, not only because of the current US administration could not convince an alligator to eat a chicken, but also because what exactly can anyone offer China?
What should also be mentioned is that now that North Korea has nuclear weapons, China has more incentive to prevent unification because what they least of all like is a nuclear unified Korea on its border.