No. He is not a heartless, and never has been.
Donald And Goofy Seperate from Sora to battle heartless in other area's of hollow bastion. That is only a prediction because there is no explanation
I'm not sure whether or not this is true, but due to the fact that Sora, Donald, and Goofy split up right before Sora fights the 1000 Heartless, Donald and Goofy probably fought 1000 each as well.
Story-wise: To effectively defeat more Heartless. Game-wise: To challenge Sora to defeat all 1000 Heartless by himself.
As Axel explained to Roxas, Heartless that are destroyed not by the Keyblade regenerate back into Heartless. However, the Keyblade has the power to release the hearts to Kingdom Hearts.
No, Sora Donald and Goofy are in a different Castle
just go down back to where the gates are and go into the door where you fought Riky, and there you should find Kairi, Donald, and Goofy
Emblem Heartless are artificial Heartless created by Ansem the Wise for his research on Heartless.
In Hollow Bastion, Sora fights 1000 Heartless alone (a gauge tells how many he's defeated during the fight), because he gets separated from Donald and Goofy. After killing all 1000, a cutscene plays.
Multiple heartless are heartless! Just like fish or sheep.
Bullies are heartless. Heartless comments hurt.
No trees are heartless.
Sora's Heartless was Pureblood, since it was neither created by Ansem the Wise nor marked with the Heartless symbol.