If I understand correctly it is the State Retirement System which
required both a divorce certificate and a "fiduciary" power of attorney.

It's irrational. 

a) The divorce certificate means nothing when it is possible to remarry 
at will (even to remarry the divorced spouse).

b) The requirement of a fiduciary power of attorney is non-sensical
where you were requesting payment to Marion as a beneficiary
and the power of attorney explicitly states your right to do so.
Classically, fiduciaries, i.e. trustees, are prohibited from 
assigning their fiduciary powers to an attorney. If a trustee
is unwilling or unable to exercise his fiduciary duties,
he may or may not have the right to appoint a successor
trustee to replace him, but he ordinarily has no right to
delegate his powers. 

I don't believe the requirements for a divorce certificate and
for a fiduciary power of attorney are mistakes. I consider them
as deliberate malicious attempts to avoid payments of sums which
are legally due, by harrassing claimants until they abandon their
claims.

Like the Massachusetts Appeals Court rulings of which I was the victim, 
this is calculated untruthfulness, this fraud committed by the state. 
There's nothing you can do about it; it's important not to spend 
(a lot of) money on lawyers, it's important not to make Marion and 
or Micha miserable by exhuming the pain of their divorce, and it's 
important not to let the dishonesty of the government make you unhappy. 
I learned my lesson.