Interesting. But I doubt South Korea would attack the North, knowing perfectly well their main cities can be razed to the ground by NK's conventional, not even nuclear weapons. At the same time, I don't think North Korea would start blasting off nukes over major cities, that would be pointless. In the worst case, a single nuke over the ocean, but close enough to a target, would be a sufficient signal of readiness to defend itself. Directing nuclear bombs to major targets as a first move doesn't make much sense, you don't leave the enemy any other option than obliterating you.

Note that the incident is started by North Korea downing a civilian jet, even if by mistake, South Korea reacting, NK going berserk with nukes, US responding with conventional weapons... Hmmm, a bit kind towards the West. The US certainly come out as the level headed guys, which doesn't sound very coherent with recent rhetoric and events.