1)
What is the significance of "Shomerei Mishmeres ha'Bayis"?
1.
Malbim: They will be in Ezras Kohanim, and will be Pasul for Avodah. They will not guard Mishmeres ha'Mizbe'ach, only Mishmeres ha'Bayis ? to rinse the floor, prepare Kelim?
2)
What does "l'Chol Asher Ye'aseh Bo" include?
1.
Malbim: They will have a chamber (Lishchos Sharim) to stand on the ground there and sing with the Leviyim who stand on the Duchan (refer to 40:44:3:2).