So I might be giving the book too much interstellar, sci fi vibes than it really has, but here's what I think so far in the most bizarre way possible. Btw I think the only reason I'm thinking so bizarre is because multiple people told me they didn't see the plottwist coming so 🤣!! It must be really crazy. Anyways.
To establish, I think this Humin is indeed from a different timeline and somehow ended up in another universe's (the conditions, physics, etc are different which explains the omegaverse aspect) timeline. The reason I think this is because this Humin doesn't even understand or fully conceptualize the idea of omegaverse (not being able to smell his own phereomes, thinking of it as a fantasy). Even if he were to lose his memories and have DID (split personality), he would still at the very least remember something as important as omegaverse as it's inherent to the world. However, on the flip side, THIS Iheon is indeed the omegaverse Iheon, aka hes from this universe and timeline unlike THIS Humin. I believe omegaverse Iheon has lived through several loops of the original timeline in order to save Omegaverse Humin--not this Humin, his OWN Humin. But I don't think he realizes that this Humin really is from a different timeline. Because if he really lived through multiple loops of this timeline, or traveled in the past multiple times to save Humin, he would've expected this new personality from Humin already. But the fact that it's a complete 180 to Iheon too, this is an anomaly. AKA non-omegaverse Humin's presence is an anomaly and preventing fate from keeping the original timeline in check.