Yes i agree with you since they changed quite a lot of the story for the manhwa adaptation as we get closer to the main story's end (bcz the one who experimented on mc was the blonde girl's father not the ml's rival), but in the novel it actually tells how ml gets to mc's world and his soul was also split so he could not stay in mc's og world for long, and the reason why mc didn't left a will for his friend is bcz his friend was there to see mc off when he left for ml's world. Ngl even the manhwa's side story ending is different from the novel's side story ending but hope this helps!
Ikr I was actually shocked that the ml's rival became the ending climax/main villain of the manhwa when there was actually more to it. But if you have some time.. i recommend reading the novel version! Its actually more satisfying to read then the manhwa ver.
Btw the novel's side story actually has R18 and also a "what if" story( just treat the "what if" story as non-existent bcz it traumatized me and just skip it bcz the ml treat the mc like a literall toy basically bdsm but non-con..)

the way they ended the series was rushed and left more questions (what method did mikael used to appear in his world, yuri didn't leave a will for changhyun so he won't worry, and how mikael having the same constitution as yuri before despite not having split soul across 2 worlds)
but when we take off those it's a really nice midieval isekai bl