A very nice, close analysis. She was working at Oxford at the same time as Tolkien, so it wouldn't be surprising if she thought hard about names and their use. I wonder if they ever talked?

Ewen also turns up as a walk-on, as Captain Cameron (I think in Almond, Wild Almond) for a whole five seconds. I was pretty much, "Huh. That was disappointing", but as you say, it shows her very deliberate choice of forms of address. I remember when I first read the sentence, "My dear Windham" in FotH and just went "Aww!" Talk about lightning bolts.

The other interesting thing about the names is their meaning. Ewen means yew-tree, Keith means (among other things) "forest-lord". Similiar, no? As for Windham - well, the family crest has a lion's head, standing for bravery, and a fetterlock, meaning someone who redeems a prisoner. Seems fate had its plans for Keith right from the start. :/