1)
What is the meaning of "bi'Shmor"?
1.
Radak: When he was waiting for the [proper] time to come to the city to fight. Also Yonason translates 'when he was besieging 1 .'
2.
Malbim: He was guarding the siege [to stop people from entering or leaving the city].
2)
Why did Yo'av put Uriyah with the Anshei Chayil?
1.
Malbim: A besieged city occasionally sends out Giborim to kill some of the besiegers. The besiegers place Giborim outside, opposite the Giborim inside. Uriyah was among them; he was one of David's 37 Giborim.
