As for the need for specialist investigators, AAIB support is available for all air accidents, but there will always be a need for investigators who understand the operating environment sufficiently to reach a properly informed judgement
Which is why the RN FAA has always had professional, but serving, AIs trained to the same level as the AAIB but able to add that element of technical and aircrew knowledge to the peculiar environment of operating at and from the sea.
With respect to BOIs the FAA system allowed no avenue for "seniors" to interfere with the BOI process....the report stood on its own merits, signed off by the Board President (normally a Commander).