Why did He say "Lo Osif"? Had he exiled them before?
Radak (Melachim II, 21:8): When they sinned, He made enemies rule over them, and they exiled them from their land, even though there was not a total exile before David.
Why does it mention that Hashem said that He would not exile them again?
'Rashi': Also this verse continues (from verse 4) the 'complaint' about Menasheh. I promised to put My name here, and not exile Yisrael, if they will not sin. He [caused them to sin, and] made the Shechinah depart, and we are still in exile due to him!
Where did He promise not to exile them?
Malbim (Melachim II, 21:8): "V'Samti Makom l'Ami Yisrael? v'Lo Yirgaz Od" (Shmuel II, 7:10).
Why does it say both "Kol Asher Tzivisi" and "l'Chol ha'Torah? b'Yad Moshe"?
Malbim (Melachim II, 21:8): The former is commands of the Nevi'im 1 and oral Torah. The latter 2 is written Torah that Moshe commanded.
