1)
Why does it say "k'Echad"?
1.
Malbim: Ezra wrote in Divrei ha'Yamim II, 29:34 that Leviyim are Yishrei Levav to become Tehorim more than Kohanim are. Now, all of them became Tehorim as one.
2)
Who slaughtered the Pesachim?
1.
Malbim: The Leviyim, like it says in Divrei ha'Yamim II, 30:17.
3)
To whom does "v'Lahem" apply?
1.
Rashi: To the [Leviyim] themselves. Malbim - the verse tells their praise. They slaughtered the Am's Pesachim before those of the Kohanim, who are zealous, and the Kohanim's before their own.
