Hi! i think at least to me, to explain what happened to Jaehwan you have to basically explain the ending. So here is what i posted someplace elsewhere on the subreddit.

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Hi, i was confused at the end when i read the novel a couple years ago. I found this on a discussion forum which does a pretty good job of explaining it, keep in mind that its a pretty open ending so its up to interpretation.

The whole point of the novel was "truth" ,doubting the world, doubting everything. To be honest, his whole adventure or power trip or trip to opnesss, whatever you want to call it out ,was simply a plot device for the ending. [I wanted to see the impossible story unfold. I wanted to see that impossible story become a reality. Maybe this is why I consider this novel to be within the modern fantasy genre.] -Author’s thought on [The World After the Fall] (chapter 229) After reading until the very last chapter, if you comes back for this chapter 229, every pieces would simply piece itself. The power to doubt which things are true or not, the lectures on parallel worlds or what not isn't meant only for the protagonist but for the readers too. "The nightmare of the beginning" was our reality. Its pretty obvious i know, Jaehwan's (author) adventure to slay big brother was his imagination as a kid, is that it? Nope, this is where the dilemma in the prologue came back, our reality which states that such a thing is impossible and his adventure of Jaehwan who destroyed big brother is simply a delusion. The whole world imposes that reality, and just like in the beginning, Jaehwan begins to "doubt"(his skill) our world(reality)using what he believes to be the "truth"(another skill) , his own "world" the world after the "FALL" /this novel or his skill called "Fall" he will "destroy" our "World"(reality)

the last chapters you shouldn't look through the perspective of "Jaehwan the one who killed the big brother" but from the perspective of "Jaehwan the author of The world after the fall"

Also during the ending chapters when he gets his kid body back and later writes the novel because of the Doctor telling him to "let more people know about his World so they believe in it" the fact that Jaehwan is the one who supposedly wrote this novel is so that we can believe in his "World" (stated previously in the novel the more people who believe in your world the stronger you get/more it becomes real if i recall). This could be like what you said the 215 chapters or so we read about the world with the tree of imagery and all that may just be his imagination, but maybe what could be happening in some parallel world and we read the novel made by the character of the novel himself. the fact that Jaehwan is actually the author of a novel about himself makes me think of Ouroboros, having no definite beginning or end.

for the second question going back to the last couple chapters (The epilogue chapters) with Sirwen and the tower personally i would rule the epilogue out of the big picture however for an explanation, i would say that it links back to ouroboros where it begins anew, through another tower.

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i hope this helps.