At this point NK has lost the opportunity for a surprise missile attack. I assume we are watching their launch sites very carefully, and any move to ready a missile for launch will result in a pre-emptive strike by the US or ROK.
Granted if they were to successfully launch an EMP warhead, it would be devastating, but I don't believe they'll get the chance (assuming, as you suggest, they have the capability). More worrisome is the fact that there are 10.5 Million South Korean citizens (and US forces) living in and around Seoul, within artillery range of North Korean batteries. One nuclear armed artillery round would be devastating if it were to strike Seoul -- and that kind of attack could come without warning.