An interesting quotation from the CAB report:
"... tension on the right wing down cable imparts an immediate signal to the aileron boost unit for a right wing down control movement. These tests also disclosed that if the forces on the boost quadrant were released, the propensity of the system would be to return the ailerons to neutral in approximately seven seconds, provided no part of the system hangs up or binds. However, the tests involving cable separation further showed a tendency of the cable connectors ... to bind with airframe structure. Because of the mechanical advantage of the system, only a small hang-up force would hold against in-flight airloads and would prevent the aileron boost unit from returning to the neutral position."
In other words, the CAB is fairly confident in conjecturing that the aileron boost did not return to neutral, but realized that such return-to-neutral was possible. In the absence of affirmative control by the crew, the flight path geometry provides reason to think that this returning to neutral was happening during the latter part of the flight.
This, along with the rudder inputs and evidence from trim settings, provides a mechanism that can explain the observed flight characteristics.
The CAB ended by concluding that this return-to-neutral did not happen, partly based on witness reports and partly on the debris recovered.