Responses
I am thinking, since this has happened in other kind of stories, that MC is the person in the novel. The novel is MC's past life. I think MC and ML reincarnated into the life MC dreams about with his mother and sister plus ML has a BFF. You can see in the flashback ML was asking about fate at the sunset camping. Someone in that life wrote about MC's tragic past life as a novel. And somehow both him and ML are back at their past lives in omegaverse.
Ohh that's a possibility as well, multiple reincarnation
So this is how I understand the plot call it a theory of you will Im starting to believe that mc is not transported in the novel but is actually suffering from split personality so basically mc and ml regressions they come from the future after they died but mc because he remembers the past all the trauma and suffering he created or had a split personality that thinks that everything that happened to mc is in the novel and that the 21 years old version purpose was to make sure they have a happy ending just like the 30 years old version wanted but on ch 40 because 21 year finds out that he was not what ml wanted 30 and 21 years old switches again because he was heart broken meanwhile ml regress and remember everything (mc world: this would explain why all the people he loves and cared about are his friends and family in the non omega verse world )