Marusya is short for Mariya (Маруся <- Мария), and indeed it has nothing to do with Alexandra. The girl was just acting out a little. When Gosha stayed unfazed, accepting this false "nickname", she made a vaguely snide remark about calling him "Vasya" then, which is also a diminutive for the "wrong" name - Vasily (Вася <- Василий).
That's true, Zhora, Goga and Gosha (Жора/Гога/Гоша) are pretty common nicknames for Georgi/Giorgi. Yuri (Юрий)as a diminutive for Georgi is a bit crazy, I guess it was mentioned as an example of weird nicknames people give him.And what is the deal with Shura, Goga, Gosha..... etc! His actual name is given as Georgi.