1)
Why does it say "va'Yosef"?
1.
Radak: It is because [also before] it discussed this matter.
2.
Malbim: There was a conspiracy in Yehudah to cast off the yoke of Malchus Yehudah, and join with Pekach. Hashem sent a Nevu'ah specifically to warn against rebellion, and informed that Malchus Efrayim will end soon, in Chizkiyah's sixth year, and then the horn of Ben David will be raised. Until 9:7 discusses this.
