1)
About what should Yo'av not feel bad?
1.
Metzudas David: That some of the soldiers died. Even the victors, some of them die in war!
2.
Malbim: That he came too close to the wall. It is better to do so until destroying the wall. Even though many die, this ends the matter. If you stand afar and prolong the siege, the enemy's sword will consume many, for they go out from time to time.
2)
Should Yo'av strengthen the city?!
1.
Rashi: "V'Chazkehu" is not part of the message to Yo'av. It is an instruction to the Shali'ach to console and encourage Yo'av, lest his heart soften.
