Why does the Pasuk repeat the words "b'nei Yisrael" five times?
Rashi: To demonstrate Hashem's love of Yisrael
Even as He speaks about them in a derogative context.
The Pasuk explains that Yisrael will not suffer a plague. Why ought they to?
Rashi: Because, if the Kohanim and the Levi'im had not been designated to guard, they would have had to approach the Kodesh, which would lead to a plague should they come too close. 1
Torah Temimah (citing the Yerushalmi in Pesachim, 4:1): We learn from this Pasuk that if either Kohanim, 2 Levi'im 3 or Yisre'elim 4 (the owners) are not standing by the Korban Tzibur when it is brought, the Korban is Pasul.
Seforno: See Bamidbar 18:3, where the Pasuk writes that, even after the Levi'im have been chosen, Yisraelim who enter the Mishkan will die, as well as the Levi'im who allow them to enter.
Torah Temimah (Ibid.): From "Ve'etnah es ha'Levi'im Nesunim le'Aharon u'le'Vanav mi'Toch b'nei Yisrael".
Torah Temimah (Ibid.): From "La'avod es Avodas b'nei Yisrael (u'Lechaper al b'nei Yisrael)".
Torah Temimah (Ibid.): From "ve'Lo Yih'yeh bi'Venei Yisrael Negef".
How will the Levi'im atone for Yisrael?
Seforno: By Yisrael giving them Ma'aser Rishon, thereby enabling them to serve Hashem in their place, now that Hashem has rejected the firstborn from serving.