Writing to anyone within the Chantry organization will eventually get the letter to the correct place, but Brother Armin and Sister Joselyn have been appointed to handle mage issues specifically, and sending it to them will get the fastest response.

Even the fastest response will still take a pretty long time—with the respondent's apologies, etc., it's the Exalted March—and Derrica will be told she can have her phylactery, but she has to come to Val Royeaux herself to retrieve it, so they can use her proximity to find it in the enormous, poorly-labeled collection they inherited and be sure they're giving the correct phylactery to the correct person.

If she decides to go to Val Royeaux to do that, let us know, and we'll give you more info about how that process will work!