"the shaft was left unsupported in the MGB". Correct me if I'm wrong (I usually am) but the schematic of the gearbox and bevel gear at the bottom of the shaft to engage with the oil pumps, in no way indicates that the loss of the bevel gear would leave the shaft unsupported. The weld holding the bevel gear to the shaft fractured and the gear dropped off the shaft. Shaft continued turning, fully supported by its bearings.
With any luck the latest ditching isn't due to the same cause, but due to loss of oil from somewhere else like a failed engine input seal. Hope it's nothing worse because I fly these truly delightful machines every working day from Aberdeen.