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There is no "statute" saying they must attend mediation. The judge can order them to attend mediation and if they fail to do so, then they can be held in contempt of court and penalized by the judge for not abiding by the court's orders.
Interrogatories can be sent to both defendants separately, you can ask for information from them that you would like to know, including names, addresses or any other information.
You also can send them Request for Production, where you can ask them to produce any documents that you need or you believe they have that could lead you to find evidence you have. You ask them to produce the bill of sale, for example.
Finally, you can send Requests for Admissions, where you can make statements of fact requiring them to admit or deny those statements.
You can ask in interrogatories if they were informed of the property belonging to you and who informed them. You want to leave it open to let them answer.
You do not need "cases or statutes" in a simple conversion case (conversion is taking someone's property). Your complaint is that it is your property and Defendant 1 had no right to sell it, converted it, and you are entitled to the money or if Defendant 2 knew it was stolen the recovery of your property.