What are "Korbanam" and "Chatasam" referring to?
Rashi and Ramban: "Korbanam" refers to the bull of the Olah and "Chatasam", to the goat of the Chatas.
Why does the Torah see fit to add "ve'Hein Hevi'u es Korbanam ... "?
Ramban #1: To stress that, in spite of the severity of the sin, since it was performed be'Shogeg and they brought their Olah and Chatas, they deserved an atonement.
Ramban #2 (citing R. Yoshiyah 1 in the Sifri): If one tribe sinned following a ruling of its Beis-Din, and other tribes followed suite, they are all subject to bring an Olah and a Chatas.
Ramban #3 (According to Kabalah): The Torah inserts both "la'Hashem" and "Lifnei Hashem", the former, corresponds to the Midas Rachamim; the latter, 2 to Midas ha'Din 3 - which Chazal sometimes describe as 'Hu u'Beis-Dino'.