IMHO if the regime collapses South Korea can not hesitate one second. They must get in and get in fast. Hopefully they will not meet Chinese armor at Pyongyang.
But they will be met with China’s armor swiftly coming over the border. China will not risk the Us on their borders under any conditions...N.Korea, to the Chinese, is simply a buffer zone from the S. Koreans rise to freedom and the 21st century. If S. Korea goes in fast China will definately do more than oppose.