If you look at the case decisions in Enron I think you will find a mid-level employee got pulled in as a fiduciary for signing a Form 5500. I don't know if she was ultimately liable
The issue we have is that the transaction appears to be for the benefit of the former trustee but not all of the facts are established, though its hard to see how the PT rules are not triggered.
Prior trustee probably committed prohibited transaction but not all facts are in so there is small chance he did not. Has either the IRS or DOL published any guidance as to when prohibited transactions must be reported.