What is the significance of the fact that here "the Tzidonim called Chermon Siryon and the Emori, Snir", whereas later (in 4:48), the Torah refers to it as 'Si'on'?
Rashi (citing Targum Onkelos) and Ramban (citing the Sifri) and Targum Yonasan: "S'nir" actually means snow - bearing in mind that the Chermon is snowcapped all the year round. In fact, it seems that the other names are also corruptions of "S'nir" (See Ramban) (and that the word for 'snow' in many modern languages - including English - is similar to it).
Ramban: In fact, 'Chermon is a nickname by which it was called because, due to the extreme cold, people tend to avoid going there - as if it was in Cheirem. See also Targum Yonasan, who connects 'Si'on' to the word 'Meisir; (sheds), because the trees that grow on Har Chermon shed their fruit (See Peirush Yonasan and Na'ar Yonasan).