IRS will not/ has not returned the money or credited my account to the appropriate years of back taxes.
We're going to need some more information here. Your Plan would have been required to pay your priority taxes in full and, if your disposable income supported it, some of the non-priority (dischargeable) taxes as well. Are you saying that the IRS applied the Trustee's payments toward older non-priority tax years before the more recent priority taxes?
Discuss with your lawyer, who knows all the facts and details. We don't know what you mean by "allowed" the overpayment to occur, why it is an overpayment, how the payment was made, when the payment was made, when you filed your case, etc. These facts, and others, are important. You are paying your lawyer to handle these questions. The Trustee does not have a duty to you.
Chapter 7 Bankruptcy Attorney
The Trustee can seek to recover the money from the IRS in the event of a mistake. Even the IRS has to follow the law. Hope this perspective helps!