I personally think all this talk of either an ICBM strike or a nuclear exchange is way overblown.
Firing a liquid fueled rocket in peacetime is one thing - on a war footing a lot more difficult - without being hit first.
What guarantee is there that even a conventional warhead on a Hwasong 12 would survive re-entry - seems there was a wide variation of views on just what those Japanese CCTV cameras saw.
They may have developed a miniaturised nuclear warhead, but have they successfully launched it and verified it's stayed intact and functioning before destroying it in flight?
How do we know that even a non-miniature North Korean nuclear weapon would even function as required in a real world setting rather than the controlled underground tests we've seen so far?
What's their quality control like?
I think what's far more likely is a short (maybe a few days to a week) really nasty conventional conflict.
A few dud Scud type shots launched at Tokyo and Guam.
The Northern suburbs of Seoul pounded for a few hours by artillery and other SK cities targeted by SRBMs.
Until they were eliminated en masse by conventional US airstrikes, and US and SK ground forces.
Would the Chinese really intervene at that point or before, or after?
Somehow, despite all the statements, I doubt it.
I don't think they'd use nukes in response to a massive American conventional strike.
And it's then that the real uncertainty starts... almost as difficult as the conflict... is what follows.