"Normally when you see an odd name like Devereux it has something to do with the mother, or her maiden name, but Emmet's mother's name was Pierson, so perhaps it does go back to the family's dealings in France."
I'm not sure I would call the name Devereux an odd name; it's just French, even if some may think everything French is a bit odd anyway.
But a name like Devereux Emmet does sound sort of odd since it is clearly of two distinct national heritages. Emmet's first name may go back to his mother's mother, or her mother's mother. Some historians often don't pay enough attention to the so-called "distaff side" (female side) as opposed to the "spear side" (male side) and the various significances of it, particularly in some national heritages.
It is not that unusual in certain national cultures and heritages for the family to take on and maintain as their surname a combination of the husband and wife, generally as a hyphenated surname, and particularly in the royal or aristocratic class (particularly Mid-European and Russian aristocratic heritages). I don't know it for sure but it is probably the result of various historic ramifications to do with inheritance and estates and so forth and so on.
I have only one full sibling---a sister. But I also have a half brother who is also my second cousin (his mother and my mother were cousins with the maiden surname of Clark). My half brother is about ten years older than me and about ten years ago he did something with his name I have never really heard of before. His real name was James W. Paul; his mother's maiden name was Hortense Clark. About ten years ago he decided to formally changed his name to James W. Paul Clark. When I asked him why he did that he said that he felt his surname had honored his father's name for sixty years and he felt like from now on his surname should also honor his mother's maiden name!