1)
Why does it mention that Yisrael tore away from Beis David?
1.
Malbim: Only Yisrael were cast off, and not also Yehudah with them, because they started to sin by separating from Beis David.
2)
Why is it written va'Yida, and we pronounce it va'Yidach?
1.
Radak: The Kesiv is an expression of Niduy (excommunication), like "ha'Menadim l'Yom Ra" (Amos 6:3), "Son'eichem Menadeichem" (Yeshayah 66:5). I.e. he distanced Yisrael from Hashem. The pronunciation is known (successfully enticed to sin).
