With that amount of arrears you are advised to hire counsel, who will address all your questions. Arrears are arrears and cannot be modified and if the custodial parent asks for interest and attorney fees these may be granted as well. There is no magic number of years of contempt triggering any abatement.
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Interest cannot be waived. A court can set a repayment schedule but the balance never disappears. With that much at stake, call either a good collections or domestic attorney. Attorneys fees can often be sought too. Do not do this pro se.
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