Regarding the use of first and last names, maybe the scanlator incorrectly used "Nanami-senpai," since that's what Yuu's called Touko for almost 90% of the series. On Page 13 of Chapter 26, Yuu refers to Koyomi as "Kanou-san" for some reason while talking with Sayaka, but she calls her "Koyomi" in the localized version. It seems a little odd that she'd suddenly refer to Koyomi formally in that one case (Koyomi does the same to Yuu earlier that chapter, but I think students are supposed to do that when talking to teachers), so the localized version seems more consistent. Maybe when the localized Volume 8 comes out, she'll refer to Touko as "Touko-senpai" in her thoughts.
Then again, it is definitely easier for Touko to switch to casual speech with Yuu than for Yuu to be casual with Touko- for example, when Touko says "Ohayo, Yuu," in response to Yuu saying, "Nanami-senpai, Ohayo gozaimasu." Maybe someday, Touko will drop a few hints that Yuu can speak casually with her, at least when they're alone.