Wikipedia reckons that "Katharine" comes from a Latin mangling of the original Greek, which is closer to "Catherine". Of course this is consistent with Ouen's belief that the latter is an "old- fashioned" kind of name. I guess there's some support there for an annoying time-travel solution to the puzzle.
For a really annoying time-travel solution, Catherine should turn out to be the daughter of the "original" Sev, born back in time somewhere after he was returned from his trial, and transported forward to give birth to her father. Blah.
Wikipedia also gives a hesitant derivation of the Greek form of Catherine from "Hekate", as well as another from a word meaning "torture". I guess the first could make you wonder whether Catherine was an escapee from the witches. Seeing the witches as an "order of monials" is a stretch, but maybe not too much of a stretch.