I would also wildly speculate that the typical mission in which the Flying Doctor engaged the aircraft was different than the typical mission in which the ADF engaged the aircraft, and that the latter put the airframe under substantially more stresses than the former.
But again, mere wild speculation.
Do you have any facts to show that the aircraft design and ICA took into consideration the configurations, roles and environments in which the ADF actually used the aircraft? What do you say was/were the likely cause/s of the in flight tailplane failures?