In your post you disregarded I. the Sino-Soviet split, which has become quite severe by now; II. the fact that NK has pre-emptively started the attack, which is important when making treaties post-war; III. and that the artillery are positioned to attack the SK forces along the DMZ, not for Seoul(the capital city is their objective to reach by infantry, not artillery). Thus the outcome may be different from what you imagine.
Soviet and PRC doesn't have to buddy-buddy. Their relationship were very bad (Mongolia had large Soviets troops tasked to storm Beijing in event of conflict). But one thing is clear. Both Soviet and PRC doesn't want to NK communist regime collapsed or weakened(Kim family can be replaced with more competent one). Even so for PRC they don't want NK to fall under Soviet. So PRC is determined to shore-up NK. Again they doesn't need to fight. It is just establish old status-quo.
They might be OK with NK condemned but they won't accept borders are re-drawn. This is during Cold War Era and one of the most sacred rule was keep status-quo and not re-draw borders. Internal regimes can be replaced but no border adjustment, specially if its country from First or Second World.
And Korean crisis will precede Afghan intervention. So Soviet intervention could be butterflied altogether.
EDIT: Because of Kim acted stupidly, my scenario involves UN troops taking control of DMZ. So some idiot didn't lead to WW III.
Last edited: