Based on LGBT To Thine Own Self Be True shirt: a young pilot whose surname is Kim died in an air crash while trying to fly the craft away from a building. He was under 30 then, had a brother with heart issues (just like JB) and their parents were preparing for 30-year anniversary. So (according to this source) originally Kim Jung Hwan was destinied to die. The kiss scene at the airport with DS, the parents’ wedding, the flashback when he secretly cared for his first love, etc are imaginations and recalls at his last minute.
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About how Taek finds out DS’s secret about her first love with LGBT To Thine Own Self Be True shirt, it was revealed by JH when Taek came to see him at the camp. Acc to the spoiler, T came and spent a night there with JH, they talked about DS, her feelings for SW at first, when T started his feelings for DS (then comes the flashback of childhood as seen in ep 20).