1)
How could this be Achaz' 12th year? In Achaz' fourth year (15:30 calls this Yosam's 20th year), Hoshe'a killed Pekach ben Remalyahu and ruled in place of him! Also, Hoshe'a ruled from Achaz' fourth year until Chizkiyah's sixth year, in which Shomron was exiled, more than 16 years. It says that he ruled only nine years!
1.
Rashi citing Seder Olam 22: He ruled nine years from when he rebelled against Melech Ashur. You cannot say [to explain our verse] that he first rebelled in Achaz' 12th year, for if so, he rebelled 11 years - five in the days of Achaz, and six in the days of Chizkiyah - but he ruled only nine years 1 ! Rather, "in Achaz' 12th year" refers to verse 3 - "Alah Shalman'eser
