1)
Why does it say "veha'Mesharsim la'Leviyim"?
1.
Radak: The prefix Lamed is in place of a Hei (it is as if it says ha'Leviyim). The same applies to "Asher la'Melech Nishan Al Yado" (Melachim II, 5:18).
2)
What is guarding "Mishmeres Hashem"?
1.
'Rashi': People may not enter Beis Hashem.
3)
For how long was all this guarding needed?
1.
Refer to Melachim II, 11:7:4:1.
