I would disagree that 25.1505 requires one single margin be demonstrated, and i would argue that the interpretation in AC25-7C is not entirely correct. the precise wording of 25.1505 today, which is amendment 25-23 and hasn't changed since 1970, is
The speed margin between VMO/MMO and VD/MD or VDF/MDF may not be less than that determined under Sec. 25.335(b) or found necessary during the flight tests conducted under Sec. 25.253.
i would link the two phrases either side of the two "or" statements to each other only, and read that as:
The speed margin between VMO/MMO and VD/MD may not be less than that determined under Sec. 25.335(b) and VMO/MMO and VDF/MDF may not be less than that found necessary during the flight tests conducted under Sec. 25.253.
That's been the historic interpretation and with the regulation being unchanged, i would argue quite strongly as Ghengis says, that the old interpretation is valid.
The rewrite of AC25-7 was not without flaws, and it has in fact further added confusion to the VDF/MDf "story" by further mixing it up with VD/MD.