OKay here's my opinion. WARNING- I did not read most of the posts, so I'll only comment on the first, original question for now.
Why are heartless called heartless even though they have hearts that are taken by darkness?
I guess you would say that because their hearts belong to darkness now, so they have been taken away from the person/animal/thing, technically making them lack a heart. This is one of those, "I thought you said Darth Vader betrayed and murdured my father... Your father was seduced by the dark side of the force. He ceased to be Anakin Skywalker, and became Darth Vader. So in a sense, what I told you is true..." Wierd analogy I know....
Why does the heart fly up in the air when we slay a Heartless?
Because once the Keyblade or other weapon destroys a Heartless, it releases its grip on the Heart it formed from, letting it drift back to Kingdom Hearts, where it will eventually go to another personplaceorthing, explaining why the org. members wanted to fill it, so one of the hearts would be forced to come to them.
This is similar to when Darth Vader becomes good again, and vanishes into the force.
Yes I am a Star Wars enthusiest. ENTHUSIEST!!! Not nerd...
