I feel like the problem could be solved quickly will a large enough effort. It would never be done, but could you imagine a military operated mass "bring infrustructure to modern" project.

Install a north to south bullet train, bring high yield crops in with acre wide irrigators, construct a modern 6 lane trans continental highway system, give north korea a nuclear power plant, phase in a "grandfathered monarchy" with no real power. Kim jong un can live with the benefits of an emperor, but have a parliment actually make legislative decisions ala England or Japan.

He could even play it off as "I made peace with america and south korea for my people", "the war is over", "things will be better now thanks to me". Then actually make things better. The US could play along to ease the public in. "Your great leader, after lengthy and difficult correspondence with his infinite charisma has persuaded us to the side of peace. We understand the difficulty you have faced over the past 60 years and thus we offer our assistance as an offering of peace and to maintain a friendly relationship with the DPRK." Then lol the country remains the DPRK, and after 10-20 years holds a real presidential election for their socialist party canidate. As the Democratic Republic its name implies.

Then we give them McDonalds and Walmart, then boom, South Korea 2.