1)
To whom did Eilas belong?
1.
Radak: It was of Edom - "Vnei Esav
2)
Why does it say "Acharei Shachav ha'Melech Im Avosav"?
1.
Rashi citing Seder Olam 19: This implies that he became king in Amatzyah's lifetime. If not, there was no need to say so!
2.
Radak #1: He died like his fathers. Just like Yehoram, Achazyahu and Yo'ash died of illnesses via others, also Amatzyah.
3.
Radak #2: Once his father died, his success began, and he returned Eilat to Yehudah.