Remember that there might be two bodies of law to meet, not only tax law but also ERISA’s title I.
Under tax law, there might be an I.R.C. § 72(p) failure, a § 4975 nonexempt prohibited transaction, and a § 401(a) failure to administer the plan according to the written plan.
Under ERISA, there might be a § 406(a) nonexempt prohibited transaction, and a § 404(a)(1)(D) breach of not administering the plan according to the documents governing the plan.
Read EBSA’s 2025 Voluntary Fiduciary Correction Program to consider whether there might be an opportunity to coordinate tax law and ERISA corrections. https://www.govinfo.gov/content/pkg/FR-2025-01-15/pdf/2025-00327.pdf
Consider whether a failure was an “eligible inadvertent failure” and, if so, what opportunities SECURE 2022 § 305(b) might allow.
Get the advice of a lawyer who’s independent of the recordkeeper.
Although ordinarily a lawyer doesn’t accept a fee from a payer other than the lawyer’s client, one of the recognized variations is payments under an indemnity obligation.
This is not advice to anyone.