What sin is the Nazir guilty of?
Rashi #1, Targum Onkelos and Targum Yonasan: He is guilty of not having been careful not to become Tamei.
Rashi #2 (citing R. Elazar ha'Kapar): He is guilty of desisting from wine. 1
Even though the Mitzvah of Nezirus overrides the sin of desisting from wine, nevertheless, the sin, small as it is, requires a Kaparah (See Sifsei Chachamim).
What does the Torah mean when it writes "Vekadash es Rosho"?
Rashi: The Pasuk means that he is obligated to begin the Nezirus all over again.
From where must the Nazir begin counting?
Rashi: From the beginning, as the following Pasuk clearly indicates.