1. To have a better chance against Xehanort when he makes his move in the real of the light
2. He sees that he has become a normal person again with a heart
3. From what we saw maybe they can see their adventures in the realm of sleep and Mickey knowing Sora is struggling decides to go and help him
4. First he is in the Realm of Sleep and second he maybe used Sora as bait knowing that Riku and Mickey would come to save him
5. Maybe against Sora because Sora was the one that stopped him
6. A new one, probably has some of the other members but some just wanted their hearts back
7. It is Lea now, got it memorized
8. Maybe Vanitas because when he was first born, his face was like a heartless with red eyes
9. Maybe but later he may hitch a ride out of there with Riku and Sora