record keeper is an idiot
have them tell you what code section allows them to apply 100% of the exess payment to interest.
Estimate the APR that would generate and then ask them to justify that as a reasonable rate of interest under 72(p)
toolkit: While they are subject to 401(a)(4), I believe they are still exempt from the current 401(a)(4) regulations, so don't feel too bad. (They are subject to a "good faith" standard in implementing 401(a)(4), but do not need to comply with all of the nuances of the regulations.)