1)
Why does it say "la'Leviyim Ein Laseis Es ha'Mishkan"?
1.
Radak (26-27), Malbim: The Torah said that Leviyim begin at 30, to carry the Mishkan 1 . Since the Ohel rested in Yerushalayim, they do not carry it, so David counted from the age of 20. Malbim - carrying the Mishkan required strength - Ben Sheloshim l'Ko'ach (Avos 5:21).
1
Malbim: This is like the Kesef Mishneh's second resolution of the Rambam (Hilchos Klei ha'Mikdash 3:7). His first Perush was, they began to sing at the age of 30; David used younger Leviyim for other matters that will be explained.
