I suspect that ATSB policy may not permit them to take a real aircraft outside the envelope. To have done testing with less than full NL trim may not have had sufficient validity - particularly whilst the fin remained unstalled.
Firstly, FGD. I disagree strongly with you, but do respect that you apply intellect & logic.
But, a) simulators are commonly used to reconstruct accident scenarios. Its clear that the ATSB didn't do this. and b) we don't know what they did because they haven't provided any details in the report (unlike for example the Partenavia report). But the SIM flying they did and the conclusions they reached are from airspeeds outside that flown by the accident aircraft and therefore, invalid.
You don't need to fly "outside the envelope" to test a hypothesis and I think the 1978 Partenavia report demonstrates this nicely. in this instance, the ATSB took a real aeroplane that was identical to the crashed one and had a pilot experienced on type fly it. The pilot flew a predetermined flight test that consisted of flying a range of different airspeeds at each of the marked trim settings. The control forces were measured at each airspeed / trim combination. When the pilot approached the limit of comfort of control-ability of the aircraft, he stopped and didn't proceed to the next trim setting / airspeed.
If this testing was possible and warranted for the last trim related fatality, why wouldn't it be now?
A 1 hour flight, 2 brake pedal force transducers, a motec logger and this topic would have been nailed and if it was the smoking gun would have obviated the need for the whole sideslip calculation mess.
The ATSB has not provided any firm evidence either in terms of statements from the manufacturer, interviews with other B200 pilots or flight test data to support its assertion that the aircraft was not controllable with full left rudder trim. If full left rudder trim can be counteracted by foot application of right rudder, then there is another factor at play (which may better fit the long take-off run which the ATSB has conveniently glossed over).
The B200 aircraft certification requires that the pilot is able to counteract full elevator trim. There is no similar requirement for rudder trim, although someone pointed to another requirement that infers this. I would question whether full rudder trim leading to loss of control of the aircraft was diligent "fail safe" deign.
The whole premise of the report is therefore based upon speculation.