Whether the FE's Performance Manual calculations was correct or not doesn't really matter in this case.
Actually, it does.
The Captain should use ALL available information to form his decision. Using the Performance Manual was one of the 'no-brainers' in this case.
A half decent lawyer could have provided a legal argument that the captain's decision not to take the advice of the flight engineer was perfectly acceptable and on the face of things, the safest option.
The Performance Manual would likely back up the Captain in this case (I can't imagine any V1-10 post-reject brake cooling schedule on a 747 calling for less than 70 minutes). Again, using the manual would likely have helped him.