1)
Why does it say "Ki Yechet'u Lach"?
1.
'Rashi' (Divrei ha'Yamim II, 6:24 : Your nation is not defeated due to any lack in Your ability to save, rather, due to their sin.
2)
Why will they thank You after they are afflicted?
1.
Rashi and Radak (from Brachos 54a): One is obligated to bless for evil (afflictions), like he blesses for good.
3)
Why does it say "ba'Bayis ha'Zeh"?
1.
Radak: Those left in the city who did not go to war will pray to You in this house for their brothers who went to war.
