Where's Kitamura Kou when you need him?
It would seem to me that the disappearance has both to do probably with both the realization and the cathartic event. I'd assume that Hinata likely died inadvertently of a drug overdose, and by that point in the baseball game he had finaly realized what he had been searching for by virtue of his slow recollection of his memories, but I guess the actual event that caps off his development is needed to actually "ascend."
The way Iwasawa and Hinata seem to have dealt with their memories and how it led them to their individual realizations probably have a greater impact on the plot than it looks. I have a sneaking suspicion there might actually be a plot-critical difference between people who claim to remember everything (Yuri), people who remember bits and pieces and recall along the way (Iwasawa, Hinata) and people who don't at all (Otonashi). I'll take that to the Speculations thread later.