If a person is sued as they were a co-signer, they must be properly served with the summons and complaint.

You must respond in the legal way and on time with, for example a motion or an answer with affirmative defenses. Failure to do so could result...

Put in writing to them showing your proof of cancellation. If that does not work: You can either sue them or if and when they sue you, show...

You can challenge the amount and negotiate a settlement. If that is not successful you can challenge the amount in court and show their sale...

Suing past the statute of limitations may be a violation of law. Generally they will not sue.

Yes probably. Check your states rules for what is exempt. Your SSDI is exempt.

File a claim of exemption for all funds that are exempt.

Proof of a debt can be done by texts, email, and even just testimony. It is up to the judge to determine what is believable.

It may be a scam. Whenever one is unsure, give no information, demand proof in writing (do not give your address or other information),...

Maybe. If they can locate you they can take a debtor examination. They can search for other assets and employment.