I've written Magus a few times while posting here and had to correct myself. Given some of the other names, I wouldn't put it past du Maurier to intend this association.
I kept wondering why Roger seemed to have no obvious association. I mean "famous spear"? I see that
Wikipedia says: From c. 1650 to c. 1870, Roger was slang for the word "penis", probably due to the origin of the name involving fame with a spear.[6][7][8] Subsequently, "to roger" became a slang verb form meaning "to have sex with", "to penetrate".
I always wondered where "to roger" came from. Anyway, this does mean that Roger now has two associations, one with the name Dick, and the other - perhaps - in his role with Isolda?