This is the book where it's building up to Madge being Robin's foster mother. She is described as taking the place of the mother who died and Robin clearly loves her and regards her in that light.
But more interestingly there is a sentence saying Madge would have liked Robin to skip the Salzburg trip but didn't want feel it was her place to say anything as the father had agreed. But her father has no problems in another book with Madge imposing discipline when Robin had misbehaved so he probably would have listened to her.
Did EBD kill off Captain Humphries simply because she wanted to make Robin a 'real' member of the Russell family and could not figure out a place for him in it?
Yes, in a nutshell! It probably wasn't so much to make her a member of the Russell household though as a member of the Maynard household.
EBD was getting all ready to marry off Joey to Jack and she wanted Robin living with them. That would have been impossible with Captain Humphreys still living at the Russells.
Such a shame! If she had waited another book she had no need to kill him off at all. During the interval in Exile when the San was closed she could have put Captain Humphreys to England because he had to have a job . Robin would have stayed with the Russells.
Robin could then have moved to Jo when Jack joined up and Joey needed company. All done and dusted and Captain Humphreys still there.
He could have gone back to the San when it moved to England or been involved with war work. If he had stayed with the San he would have lived in or got a little cottage near at hand and Robin could have stayed with Jo visiting her father occasionally.
When EBD then had no further use for Robin, her home could then have been with her father.
Alternatively, Mrs Venables could have stayed well and married him but then unlikely Robin and Daisy would have stayed with Jo - at least not in the holidays.