Lot of rumors over past year of Kim being physically bad-off and that fake Kims are being used for ceremonies. His son is presently eight, and any chance of him assuming the role of Supreme Leader is at least ten years away.
But on your suggestion of a huge mess....various defectors have admitted that arriving in the ‘west’ has been a serious problem to adjust to. To just say overnight....South and North Korea are unified....I don’t think is possible.
There can’t be a sudden reunification, imo. Although, China would push for that, just to mess around with Korean politics.
I would probably install an interim government to take control of the country and install small changes along the way until they become compatible before any unification can be formalized.
This is a three generation project, imo.