John I'm sure you are right however as you rightly point out "The talent to formulate a corrective redesign rests with the OEM." The AAIB's role isn't to rebuild EC's 225 helicopter, its to report on the accident, or in the AAIB's own words:-
"It is responsible for the investigation of civil aircraft accidents and serious incidents within the United Kingdom. The AAIB is also frequently called upon to assist with military and overseas investigations. The Chief
Inspector of Air Accidents is responsible directly to the Secretary of State for Transport."
EC's work into a long term fix is there own business and can follow their own theories in that regard at will.
What surely must remain independant (which means AAIB or persons contracted by the AAIB) is the work done to validate the cause of the issues - otherwise for the obvious OEM's the AAIB becomes an irrelvance.
One might be minded to highlight the factors that allowed the 2nd accident in October because of guidance from the OEM to a batch of shafts. The OEM also guided the HUMS strategy which when you look at the available data from the May accident doesn't seem that smart.
The AAIB may not have permenant resources that would be effective across all specialities but independant resources do exist and to not engage and / or direct them with an eye to their own stated mission and frankly common sense is a failure of duty.