1. Yes, you will each have to pay a filing fee. The amount may be less, depending on the amount at issue in the case.
2. You can mail the answer to the plaintiff or the plaintiff's attorney.
3. You only need to file an answer. Do not file a declaration.
4. You don't have to put in any exhibits to your answer.
5. You don't have to file any declarations or exhibits with your answer.
6. You, and your witnesses, including the real estate agent, must be in court to testify in person. Declarations don't do anything.
7. You would bring all of your evidence to court at your trial.
8. You are the counter-plaintiff in a counter suit against the plaintiffs.
9. Yes, cross complaint.
10. Yes, same case number.
11. You'll have to serve them by proper service. Hire a registered process server to do that for you.
12. No declarations or exhibits.