1)
Who are "ha'Nofelim"?
1.
Rashi: They are people who heeded Nebuchadnetzar, and came out to him from the city 1 . This is like "b'Lechto El Tziklag Naflu Alav mi'Menasheh" (Divrei ha'Yamim I, 12:21).
2.
Radak: They came out to him before the city was conquered, and handed themselves over to him, like "El ha'Kasdim Atah Nofel" (Yirmeyah 37:13).
1
Radak: This is like the Targum - they obeyed him.
2)
Why does it say "ha'Melech Bavel"?
1.
Radak: Sometimes there is the prefix Hei ha'Yedi'ah (to identify) even though it is Samuch, e.g. "ha'Aron ha'Bris" (Yehoshua 3:14).