1)
What is the meaning of "lemiba'Rishonah Lo Atem"?
1.
Radak: You were not [carrying it] when we brought it from Avinadav's house, therefore Hashem struck us. Malbim ? Ha'alisem (in verse 12) applies also to this (you did not carry it).
2)
What is the meaning of "Lo Dereshnuhu ka'Mishpat"?
1.
Radak: Hashem's statute is that Leviyim carry it on the shoulder, via poles.
