1) As you said, Anti-Form is likely nothing more than a way to make people not spam the forms. It's probably a throwback to when he was a heartless for 10 minutes, but likely has nothing to do with the story.

2) I was very close to beating Xaldin. I went into a form (I think Master, not sure), and then I went Anti-Form. And then Xaldin started his chain of attacks where he's invincible, and spammed the hell out of them. Mickey had to save me.