Beatty's opportunity to restore naval control of aviation

Was Beatty First Sea Lord around the Washington treaty period, and could he have changed the Fleet Air Arm arrangement?

Yes. The panel distinguishes the 1918 decision before his appointment from an opportunity to change the arrangement in the early 1920s.

The follow-up checks whether Beatty was still in office around the Washington treaty period. The answer places him there during his seven years as First Sea Lord. It distinguishes the 1918 decision over naval aviation, which preceded his appointment, from an opportunity to change the arrangement in the early 1920s. The proposed counterfactual intervention therefore concerns his later choice to leave it in place.