So in sum, we have a non-DAL A system integrated with a secondary control system with no redundancy, monitoring, crew warnings or a dedicated means to disengage. Strap it to an aircraft where the trim system is more powerful than the primary controls and give it authority to move the stab at a far faster rate than normal trim and give it no limits of travel other than mechanical end-stops. If it goes wrong, force the crews to disable all powered stab control and equip them with manual trim wheels that are difficult and slow to operate at the best of times. Modify said wheels to make them slightly smaller and insert a more powerful damper to counteract the spring and backlash effect of the stab being moved at a faster rate than ever before - requiring an undocumented increase in physical effort. Finally inform the crews of this system's existence and get the company test pilots to retest all of this after a quick fatal crash, but do so only at a relatively slow speed. Second accident crew become the unexpected test pilots collecting a datapoint that suggests the trim wheel is impossible to move when at a higher airspeed. Cumulogranite awaits.
Apart from the flight envelop defining the configuration, CofG, AuW and airspeed/mach beyond which the manual wheels cannot be moved at an effective rate, what are we missing?