Dare I speak about...

Taemin had a dying body but desperately wanted to live. Siwon had a healthy body but desperately wanted to die. So call it fate or divine intervention but their souls switched so they both got what they wanted. I appreciate some of the ambiguity, since there is much about what happens after death we don’t actually know.

"I appreciate some of the ambiguity, since there is much about what happens after death we don’t actually know" I totally agree. And it works with your hypothesis, for sure but "Taemin had a dying body" "Siwon had a healthy body",... I don't remember anything that was saying this, where was it?
And so with this and how Min Siwoon speak about it http://www.mangago.zone/read-manga/geu_kkeute_issneun_geos/uu/geu_kkeute_issneun_geos-chapter-18/39/
It looked to me Siwon just decided to give up, nothing else and so he "killed" Tae-min's body (and I think without being aware of the situation). But well, nothing's sure, I just supposed this, so to me the fact their souls switched doesn't make sense.

I believe Woojin believed/knew that Taemin would at least regret dying if he saw him, but the complete lack of regret and desire to fight to live was missing from Taemin’s eyes. That doesn’t mean Taemin’s body could possibly live, but the desire and fire to live should have been there, especially after seeing Woojin, who knew he was always Taemin’s ‘exception’. If Siwon could just will himself to die, he would have. His strong desire for death seems to be the only thing keeping him in a coma. http://www.mangago.zone/read-manga/geu_kkeute_issneun_geos/uu/geu_kkeute_issneun_geos-chapter-2/4/
So I believe their souls switched so they could fulfill the other’s desire. Of course, there also has to be a strong element of fate, since Taemin dreamed about Siwon’s rape and suicide attempt, before he was hit by a truck.