No one expects FAA to keep expertise on staff. Their use of Boeing staff is bizarre.
In commercial construction, the authority can require additional or adjunct expertise at will; it does so utilizing independent sources, to avoid obvious conflict. And the engineering is underwritten by the builder.
And it has been that way for years...
Nothing Boeing does is so mysterious or proprietary that they must become the authority on duty of care. Or what constitutes regulation, or best practice.
It is inexcusable in aerospace that it should be this way.