Did Yo'ash truly give all the gold in Otzeros Beis Hashem?
Radak (18): It was only the majority, for in the days of his son Amatziyah, Yeho'ash Melech Yisrael took the gold and silver found in Beis Hashem in the king's storehouse 1 .
What is the proof from there? Perhaps after he gave everything to Chaza'el, he (or Amatziyah) acquired more and put it in the storehouse! (PF)
What is the significance that he gave the gold in Otzeros Beis Hashem?
Malbim: This shows the results of sin (after Yehoyada died). Even though he had the merit of striving to increase Kodshei ha'Bayis, an Aveirah extinguishes a Mitzvah (Sotah 21a), and he needed to give all the storehouses of Hekdesh, and also of his father, and give them for a bribe to Melech Aram.
What is the meaning of "va'Ya'al me'Al Yerushalayim"?
Radak: He refrained from coming against Yerushalayim.