Jon--
Thank you for the superb analysis of exactly what the problem seems to be.
That said, kudos also to Boeing for its current transparency on this issue. We need to remember that in 2006 when the A380 wingspar failed at 1.46 times Limit Load, Airbus essentially blew it off...even though they eventually did go back and make appropriate reinforcements. And when the A330 rear wing spar failed at about the same stress level in static testing in September of 1992, not much at all was released. Try finding details about that one now.
It is perhaps instructive to note that use of major load-bearing composite structures at this level is new for Boeing and still in early stages of experience even for Airbus, who began using composites more extensively much earlier. Combine that with the fact the the failure comes along a line where two different structures manufactured by two different corporations meet in a high stress area. Some obvious questions: did the two Japanese companies do a join and stress test at the component and substructure level? Was Boeing in on that process? If it did not happen, why did not Boeing mandate it? And if it did, what were the results at that time? Problem surfacing here may therefore be another in the series of what happens when you outsource design and manufacture of differing component parts to other companies half a world away and do not supervise scrupulously at key development junctures.
Lee