This is an interesting take on what could have happened after they returned to the island. Hm...smoking seems a little OOC, even if you did explain it, but an original idea never the less. One major complaint though: The bold. Why is it all in bold? If anything, it takes away from the story, and it's really annoying to read. I would suggest you put it in normal text.

very very good
i like
at first i was afraid it'd be too preachy
liked the last two parts, very very much
bravo

Overall, I like your story. It has a darker tone than the "happy-go-lucky" predictability of KH. It presents a scenario where our heroes and...heroine (for lack of a better term) have been affected by their experiences in a negative way. Instead of the uber-positive spin placed in KH, where they take these experiences, and after about five minutes of sadness, recover and take it all in stride, you address the fact that these kids were adolescents who lost their home, their friends, their normal childhood, and had to fight to regain it. You've played the game, you know what I mean. (lol PMF) So in short, you've placed some reality in the outcome of the character's lives, free from Disney's touch of innocence and rainbows.
Part 1- Don't take this the wrong way, but when I first started reading it, I thought that it was going to be like a PSA about the dangers of smoking. I'm happy to say that the thought was quickly erased from my mind. I love the character development. Like I said before, a reaction to difficult circumstances. There is a tendency in some stories to ignore the fact that events like these should have an impact on the personality, the spirit of a character, and just portray them as heroes who, at the end of the day, get their happy ending. I'm so happy this is not one of those stories. Great use of foreshadowing at the end, btw.
Part 2- Here, I love how you establish how/why they changed in such a short section. It's minimalistic; just enough info to inform, but no filler.
Part 3- I friggin love this part. It's great. Here you totally bring in the manifestation of Riku's actions. It fits right in, due to the nice ending you had in part 2, which let the reader put 2 and 2 together. And again, the realistic implications of these events. Riku's reflections on his actions, his growing loathing becoming fatal...I just loved it.

Few things on grammar, diction:
-when you use the rhetorical fragment, click. I like the use of it, but I don't think it should be in quotations. It's just , Click.
-You misspelled inhale in the last line before Part 2.
-When you say, "something that made her pure, it was innocence." It feels too rushed to me. I know run-ons can be used for certain effect in writing, but here it sounds much too rushed. You might want to change it to 2 seperate sentences.
-Her mind started "loosing" into other things? I think you meant "losing," as in she began giving into other impure, non-Disney related thoughts.
-Did you mean the stress OF something, not the stress for something?
-Just some random mechanics rules of putting commas before the quotes, I'm sure you know them, but frankly, it's really not that important, the story's meaning isn't all that changed.
All in all, not too many things that need some changes.