Does Henderson's possible influence during the Suez period assume that he would still be serving in the Navy?
Short answerNo. The scenario only requires him to be alive, with the discussion calculating an age of about 74 in 1956.
NotesThe clarification distinguishes being alive from continuing to hold a naval post. Using his stated age of 57 at death in 1939, the discussion calculates that he would have been about 74 in 1956. His possible influence during Suez is therefore imagined as that of a surviving senior naval figure, including after retirement.