Actually, you might be onto something... i also had this theory about him being able to coexist with his younger self cause he "stole" his brother's identity.. thus the brother in 2nd timeline being MIA because he literally became the "brother" when he used his name...
You also said something about the clock in the beginning. Remember it verified him and confirmed his identity?? But the name is blurred out?? That might be true being the "name" of his brother. Some spoilers did say he looks exactly like his older brother maybe that's why it recognised him as his brother?? Idk, I'm also theorizing why his brother in the 2nd timeline doesn't exist or why he's MIA but that was never clarified even in the novel
so to novel readers, please don't clown me when i turn out to be wrong lol. But if i turn out to be right, spoiler alert!
So, it turns out our mc has a brother that he hadn't met in his time. There was insinuation on who it could possibly be, might be the one with the crow/plague mask thingy because of how he was described as somewhat kind with the kids.
Remember the centipede boss that came out of the rift that ruined the world?? When the monster met our mc, it kinda disappeared only to leave behind a clock that turned back time for our mc??? Suspicious!
It was shown that the evil organization was messing with the dungeon that has centipedes in it, so what if either the older brother or the bad guys made the clock (since our mc said items aren't normally dropped from monster remains), the brother took it and did something so that when the time comes, the clock will only show itself to our mc and the bad guys found out and killed the brother, so that's why he didn't show up at all??
Kinda farfetched, but i have read other manhwas with siblings in them and this isn't so farfetched considering... Idk, i might be wrong tho