Who might Henderson have prepared as his successor?

If Henderson had been able to prepare his successor, whom might he have chosen?

Probably Fraser, whose seniority, position, and engineering interests made him a plausible choice if time had been available to train him.

Fraser is a plausible choice because of his seniority, position, and interest in engineering. The missing condition is time for Henderson to prepare him. Roughly six months of training is suggested. The succession is described as unusual because the Third Sea Lord's responsibilities passed on without the normal preparation.