Also, am I crazy or did they show that Liv's father had some kind of weird metal head under his wig? I seem to remember them showing that earlier in the season.
Yeah, that was shown but not really explained. But presumably the idea was that, since the only thing that can kill a zombie is a shot or other injury to the brain, he had the metal cap implanted to cancel out that weakness.
Also when Frenchie BadAccent shot him a few episodes ago I thought there was a "clang" noise and his "death" was meant to be a fake out. But I guess not since he never showed up again.
You're partly right. Enzo shot Martin in the head from behind, and was surprised when he survived because of the metal plate. The repentant Martin then turned to Liv on the open Skype call and tried to tell her something vital, and Enzo shot him again at point-blank range (or maybe in the face to bypass the metal plate?) and killed him with Liv watching.