It is a question of "what wold a "prudent expert" do in similar situations." That, in my mind requires an analysis of the costs and benefits of having a procedure that - by definition - would create errors, but also would keep costs low.
I think the fiduciary would lose. The "cost" of the plan sponsor/fiduciary performing their duties correctly is never a consideration in determining whether the action was appropriate. Only costs to the plan would be an appropriate consideration. Per the OP's original situation, the "costs" to the fiduciary of actually being a prudent "expert" was nil. "RTFD" - in this case, "read the ... document" and make a decision (apply your best fiduciary judgment) Takes some time, but no costs.