Hrm. So far, it looks like most of the official copy refers to Nana K. as "Nana," but Nana O. and gang always refer to her as "Hachi." Though Nana K. has protested this several times. But yeah, Viz calling her "Hachi" in the summaries and back cover copy is totally a Viz thing, because all the summaries and back cover copies of the actual manga has "Nana."

(sorry, took me a while to remember to check)