The way I see it the cause of the failure was a fracture resulting from a source that did not produce detectable spalling. If the whole design philosophy of MGB's is dependent on the early detection of spalling to warn of impending failure then the fact that no spalling was detected (if it was ever produced??) then we are not in a very good place and gearbox designers must be quaking in their boots. Mind you as a non-engineer my poor opinion of using a gear wheel as the outer race may not count for much. What might count a little more is the fact that there were two 'qualities of gear wheel out there but no one outside AH appears to have known about that rather interesting and worrying fact.
G