North Korea is so poor and desilate from UN sanctions that they prolly have a few months fighting ammunition without aid from china. The South Korean ROK is well trained and would smack that ass of NK. If China got froggy is when we would step in. I wouldn't be surprised if Russia didnt aid them behind the scenes or straight up join the fight.
What you may be forgetting is that if N. Korea decided to launch it's nukes, they are just minutes away from city centers with millions+ of people. I don't think China and the US want to fight, especially over something as silly as North Korea. If shit got fucked up, China would have millions, and millions of North Koreans coming into their country for refuge. When they get there, they would realize that for the entire lives (could be 50 years, 20 years, etc...) they have been lied to and brainwashed. If that were to happen, it would be a huge outrage. North Korea is a little shit of a country, but they managed to get very very few leverages, IE nuclear weapons. When a country has nuclear arms and is as fucked up as North Korea, you basically have to comply to their demands or else they could fuck shit up.
Yea, i've seen that too. They dont have warhead devices to secure a nuke. Their missile has no "POW!" to it. US and ROK are constantly "drilling." They can take a bollistic missile out of the air without detonating.