Personally, with the revelation of more and more issues as you peel back the layers, I'd send the client to ERISA counsel. If they need document/admin/termination support, have the attorney engage you on behalf of the client. This is for the client's protection since there are multiple IRC, ERISA and fiduciary issue in play, best that the client is communicating under attorney-client privilege. Don't forget there are potential claims from employees, not just the agencies.
It's also for your protection. IME it's almost impossible for a TPA to get properly compensated for the work/risk/complexity involved here. People expect to pay 5 and 6 figures to lawyers, they expect to pay 3 figures to TPAs.