1)
Which people will be added?
1.
Malbim: Those who returned in the days of Koresh and in the days of Artachshasta.
2)
Why does it say "Kol Beis Yisrael Kulo"?
1.
Radak: The repetition is to strengthen the matter, and teach about [the return of] the 10 tribes, which were hidden until today.
3)
What do we learn from "v'Noshevu ha'Arim vehe'Charavos Tibanenah"?
1.
Malbim: The uninhabited cities will be settled, and the destroyed cities will be built.