I think you are technically allowed to sue anybody for anything. That does not guarantee it you will win the court case, however.
Sue them for wilful noncompliance and harassment.
No you can not
Yes.
Yes.
YES
Yes.
Unless you have a specific repayment plan that the collection agency agreed to, there is no legal reason that cannot sue the cosigner.
Any collection agency can, in theory, sue you anywhere they can find you. Whether or not they have jurisdiction or a valid claim and whether you have defenses would depend upon the circumstances.
yes
The best way to stop a collection agency from contacting you is to write them to stop collecting you. It really is that simple after you write to them they are required by law to stop calling.
If you are not refusing to pay and you pay the debt, they would have no reason to sue you. If you refuse to pay a valid debt, they may advise their client to sue you depending on what state you are in and what the laws are there. Some states only allow original creditors to sue and not the collection agency.
yes