1)
What Sechel did Zecharyah use?
1.
'Rashi' (Divrei ha'Yamim II, 30:22): Sechel applies to Shirah, like "ha'Maskilim Sechel Tov la'Shem" (ibid).
2.
Malbim: (It is keen intellect.) Shelemyah had seven sons. The eastern gates fell to his portion - there were only six, so one son must be pushed off to elsewhere. If another son is pushed off, he will be shamed. Zecharyah was the Bechor, so there is no shame for him - people will not say that he was pushed off due to inferiority!
2)
Was only Zecharyah in the north?
1.
Malbim: No. There were four gates in the north (17). Since the lottery already fell on the north, three sons of [the next for whom the lottery was cast, i.e.] Oved Edom guarded there.
