Any missiles the North Korean's fire on Seoul would likely be intercepted by the THAAD systems that were installed within the country, and Japan has the Aegis BMD system in the event of a ballistic missile attack. Even if the DPRK could succeed in getting their missiles off the ground, they would be quickly shot down. It is also clear that South Korea would be able to defend against an invasion across the border and quickly retaliate and send troops directly towards Pyongyang.

China probably isn't going to help the DPRK, that wouldn't sit well with their interests abroad in terms of U.S. - China relations and trade. North Korea has been backed into a corner and it is likely a much smaller threat than ever as China begins to distance itself from the North Korean government.