Yankelberl, thanks for the “Rav” suggestion. It would surely apply to first/last names like “Rav Avigdor Miller”, but not to pen names like “Chofetz Chaim”.
Chaim Volozhiner seems like an intermediate case, Volozhiner being sort of a nick name. I consulted reb Google, and it seems that appellation is more used than not, so your correction seem to be more correct than not. I also notice that he was also known as _Reb_ Chaim V… Maybe we can follow psak of R Gershon Kamenetsky who puts R for everyone and suggests to read it as Reb/Rav/Rebbe/Gaon Hatzaddik according to reader’s preferences.