FBW, fair enough, but you have to assume first that you have a lateral moment, or why should it break sideways if the moment is fore and aft ?
Further more, IF we assume only accelerations and not aerodynamic stresses, most of your argument goes down the drain, leaving us to guess at where the structure failed.
Mind you, I would tend to agree with an in-flight break-up and lateral streesses on the vertical fin, I'm just playing the devil's advocate for the sake of prudence.