Grumbler, I had the same concern about short-ranged anti-seeker seekers. However, on testing them in combat I found they worked just like any other point-defense weapon.
Re your question about AMM 5 replacing RFAMM 4 in the design window when you have both available? Take a closer look at the technology requirements of the AMM and RFAMM.
Name := Anti-Missile Missile I
Number of Tech Req := 3
Tech Area Req 1 := Ballistic Weapons
Tech Level Req 1 := 1
Tech Area Req 2 := Earth Alliance Heavy Weapons
Tech Level Req 2 := 1
Tech Area Req 3 := Point-Defense Weapons
Tech Level Req 3 := 1
Number of Abilities := 1
Name := RF Anti-Missile Missile II
Number of Tech Req := 3
Tech Area Req 1 := EA Advanced Ballistic Weapons
Tech Level Req 1 := 1
Tech Area Req 2 := Earth Alliance Heavy Weapons
Tech Level Req 2 := 1
Tech Area Req 3 := Point-Defense Weapons
Tech Level Req 3 := 1
Number of Abilities := 1