Why does it repeat that David left? Verse 16 already said so!
Malbim: Verse 16 discusses leaving his house. Our verse discusses leaving the city.
What is the meaning of "b'Raglav"?
Radak citing Targum Yonasan: It is 'with him.' This is like "v'Chol ha'Am Asher b'Raglecha" (Shemos 11:8).
Who is "ha'Am"?
Malbim: They are mentioned in the coming verse.
What is "Beis ha'Merchak"?
Rashi citing Targum Yonasan: It is a distant land [Radak - from Yerushalayim].
Radak citing Midrash Shmuel 8:2: [Hashem] excommunicated David 1 . "Yashuvu Li Yere'echa" (Tehilim 119:79) implies that they had separated from David.
Malbim: It is a house far from the city, to gather there.
Midrash Shmuel 8:2: David accepted this, and his head was covered (verse 30).
