No, you would use the first if you didn't remember the person you are speaking to but they remembered you. The second would be used if you thought you had met the person you were speaking to but weren't absolutely sure.

'We've met before?' isn't really a question. It's a statement with a question mark at the end. The correct form is 'Have we met before?'. You would say 'Have we met before?' if another person appears to think that they know you, but you don't know who they are. In fact, it's a more polite way of saying 'I don't know you. Who are you, and why are you speaking to me as if you know me?' You would say 'Haven't we met before?' if a person seems very familiar to you, but you can't quite remember where or when you met. You are hoping that the other person will remind you about how you know each other. Haven't we met before?

yes they are.

Yup. You can also say "Have we met before?"