I was lucky enough to be stationed at a forward RoK marine base back in the 80s. I can't say where, but we could see the occasional tracer or star shell come down from the north. The RoK Marines are a nasty, ready, no BS group of guys. I could easily see a 5:1 or better kill ratio in combat with the north engaging the RoK Marines. And, notwithstanding any cultural, or racial elements the RoK guys really hate the NK.
Furthermore, we are pretty well prepared along the DMZ. Sure, there's going to be some battle damage in the first 48 hours if things go pear shaped, but after the first 2 days of live action, things will be pretty one-sided. If there are carrier assets nearby, the rear guard of the NK forces are going to be in for a very bad couple of days. The civ losses in SK and around the Seoul area will be bad. I won't lie, it will be a mess to clean up, and plenty of casualties. But - I honesty think that the retaliation won't stop until there is nothing left of Pyongyang, and the Wonsan naval base.
Internationally. China won't move. They might bluster, might rattle some phones, kick a ambassador out, but they will NOT pick up arms. The US is by far their biggest trade partner, and lack of access to the US market at this point is unthinkable to the govt of China.
Russia might send some weapons and/or materiel support, but they ain't gonna send any bodies out there. What's the upside for Russia to support NK? It's a foregone conclusion they will lose against the SK and US, so why stick your neck way out?
Back in the day, we didn't worry much about the NK air assets. Their fixed bases, no matter hardened or not are so vulnerable to AWACS intercepts, that anything getting off the ground from NK should be picked up fairly fast, and dealt with by surface assets. If not, we have a host of fixed wing and rotary wing support near by that very little is going to get through the DMZ.
I"m guessing here that things haven't declined much if any since I left, and more than likely things are being reinforced as we speak. The fat boy up there is no idiot when it comes to self preservation. He has to know he's targeted by multiple systems from the south at all times. I would be concerned if we DIDN'T see him out for a few days, indicating he's gone to ground somewhere. That would worry me more than all his fist shaking and finger wagging.