Why was this called Chet ha'Ne'arim"?
Kli Yakar (Lani'ado) citing Mahara: People say that the youths are Bnei Eli. Really, they are young Kohanim. Their sin resulted from seeing ha'Anashim, i.e. Bnei Eli, despise Kodshim.
Kli Yakar: The youths are the Kohanim's helpers. To Anashim (Yisrael), it seemed like a simple Chey. "Es Pnei Hashem", Who knows what is in the heart, it was a great sin of disgracing Kodshim.
Malbim: The youths are the Kohanim's youths. Chet is Shogeg 1 . Indeed, one may roast the meat before Haktaras ha'Chelev; only eating before Haktarah is forbidden. However, they have no right to take it before Haktarah. Even so, it was a great sin, for it made people disgrace Kodshim.
Indeed, we usually say so. However, Chet often refers to Mezid, e.g. regarding Kayin, Sedom, Pharaoh, Dodaso, Terumah, blasphemy, in the Tochachah
What is the meaning of "Ni'atzu"?
Rashi, from Targum Yonasan: They disgraced [Radak - and despised].
What is "Minchas Hashem"?
Radak citing Targum Yonasan: It is the Korbanos of Hashem.
