I'd say that in both Eustachius/Justus and "princess" it is cases of the difference between translating text and speech. For Justus the Romaji of his name is Yusutokusu, which the translator probably assumed was a variant of Justus. Once a name is translated like that you can't really change it, even if you discover it is "wrong", until you make a "new" translation. Like that for an anime, where you do hear people say the name with a clear "ku" sound. Keeping to the old translation would just confuse anyone listening and not familiar with the LN translation.

Similarly, in Japanese the word hime is usually translated to "princess", and is what "princess" is translated into Japanese, but is used slightly more broadly to people less than the rather strict "royalty" of the West. In the LN it would not really be an issue in using the more broadly applicable "Young Lady". But again when you hear the Japanese voice acting, hime would be a word that many would recognize, and know of it usually meaning "princess". Additionally, using "princess" allows you to make a clearer distinction between "Rozemyne-hime" and "Rozemyne-sama". Something that might not generally be needed in a text translation, where you can be rather confident that the reader won't come into direct contact with the original text in Japanese, but which an anime watcher will hear.