1)
What is the meaning of "Kamu Einav"?
1.
Radak #1: This is like "Einav Kamah" (Shmuel I, 4:15). In old age, the fluid [in the eyes] dries up, and Yisri of the eye are Kam (rise), and the eye enters the hole.
2.
Radak #2 (Shmuel I, 4:15): His eyes pined from longing.
2)
What is the meaning of "mi'Seivo"?
1.
Radak: It is like mi'Seivaso (due to his old age). Also "v'Tzuram Livlos She'ol" (Tehilim 49:15) is like v'Tzurasam, and "ki'Svunam Atzabim" (Hoshe'a 13:2) is like ki'Svunasam.
3)
Why did Achiyah's eyes weaken?
1.
Radak citing Bereishis Rabah 65:10: Anyone who teaches an improper Talmid, his eyes weaken; we learn from Achiyah. Yaravam was his Talmid.