Does the return of stolen goods constitute proof of theft in Islamic law, thus permitting an oath to be taken on it, especially since the law of the land considers the return of stolen goods as proof of theft?

Theft is not proven except by the thief's confession or the testimony of two men. The presence of stolen goods with the accused does not permit you to testify against your wife for theft. Rather, you should testify that she returned the stolen items. Testifying to theft without your direct knowledge is invalid and forbidden. Testimony is only valid concerning what the witness knows.