1. Could you tell me what you mean by shadow creature. If you are talking about neoshadows they only appear in chain of memories and kh2. They are everywhere in the deep dive trailer.
2. He turned evil after darkness had consumed his heart. He used to be the leader of the people who lived on hollow bastion. Cloud, cid aerith, Leon (squall), yuffie, and the rest of the villagers. I think kairi is somehow linked to ansem because she first lived in hollow bastion. Ansem was a man who wanted to understand it. His discoveries led to his own demise after darkness had consumed his heart mind and body.
3. This one is a bit hard to explain. Well ansem opened the door to the darkness but the heartless were already there. Before he was consumed by darkness heartless and darkness was something he deticated his life to studying.
4. No, he is simply tossed to a random planet. You would know this if you watched the little clip at the end of the credits.
Lets live together, I don't know how long it will last, but it doesn't have to be for show, together, till death do us part. - Kenshin "Shinta" Himura - Rurouni Kenshin