see this is the situation:
NK has no less than 500 HUNDRED artillery batteries aiming directly to Seul, If shit goes down, SK WILL have to attack first to get the upper hand. that means a combined strike of tanks rolling into Nork land to keep them busy and aircraft storming the sites. keep in mind that while the SK forces are decades more advanced and are better trained, they are outnumbered 1 to 4.
and this is where the nukes kick in. i really dont know about that...
i doubt China will risk their economy, the US is their biggest market and could definitely not sustain themselves alone. they depend on selling crap. they will probably trash talk if America intervenes but wont do a thing. Russia neither.