3. I used both in case one word was familiar to you and one wasn't. I think disposition might be, in some cases, a little more temporary than personality, which you're really born with.
1. I wouldn't use "mild" to describe a girl, but I might use it to describer her personality.
2. It's hard come up with a single opposite. I suppose the opposite of "gentle" could be "rough," (or bruitsh, or harsh.)
The opposite of a mild-mannered person could be one who is forceful, assertive, aggressive, in-your-face.. there are so many.