
As someone who doesn’t believe in soulmates/fated lovers irl, the other string linked to the girl makes sense to me. There’s probably countless people that are compatible with you, but only one person you choose to be with (unless you’re poly). So the strings could be showing him other people he is compatible with/divergent paths he could take, but in the end, he’s still going to choose Kihoon because that’s who he has built a relationship with.

I read the ending spoiler, not sure if it’s true or not, but even if one of them passed away, this wouldn’t be a tragic story for me. Death is an inevitable condition of human life. Ki-chan and Shuji had 30 beautiful years together, full of cherished memories and comfort, this manga is a celebration of their love and life together.

Even if Cheongsun had found the letter 10 years ago, the most they could’ve had was a short long distance relationship. 10 years ago, Cheongsun was very insecure about his class and family status, it’s the reason he broke up with his first girlfriend. He saw Gonghyun as living in a completely different world than him and eventually it would’ve eaten away at their relationship at that young age, especially with the stressors of school and finding a job. They both needed the 10 years to become more financially independent and self-assured. The reason journal was only shown at the ending is because it doesn’t matter that much in the plot, it doesn’t change things. Now that they’re finally together, they have the rest of their lives to tell each other secrets and feelings they couldn’t have expressed when they were younger.

This was already posted as part of the Bara Boom Anthology, chapters 5-7: https://www.mangago.zone/read-manga/adult_bl_shorts_bara_boom/
The character’s name is Shouta, which is a common male Japanese name. It’s not so much a twist, since it’s obvious from the name alone, but it’s a cute story of two friends going to a café together where Mii is very supportive of his friend’s hobbies.