After all the excellent information about duplex bearings and preload offsets , we are still left to find a reason for the bearing failure and the excess torque on the spider end nut.
Unless the split pin at the spider end had come adrift, the control rod can't have rotated relative to the nut to tighten it - is the initial report wrong when it says that the torque is a result of that rotation and if so, how?
Unless there is a clear explanation of that process you come to the conclusion that the nut had been overtightened either on assembly or during servicing. Whether that excess torque directly affected the operation of the duplex bearing or just exacerbated an existing problem with it is one of the questions the AAIB will hopefully answer.
Surely the bearing failed first?, so the inner race turned on the shaft, tightening the nut and shearing the pin? A bit of extra torque on the nut would not make any difference to the bearing clearance unless it deformed the inner races, which is unlikely. As above, the clearance is set by the machining process, the nut only has to apply enough pressure to hold the inner races firmly together.