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. 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.
I had a similar thought. Not so much the network forbid a proper death but that they left things intentionally vague in the event that a revival does happen at some point down the road IIRC when Dolly was plotting with her followers at the start of the episode there was talk about a bomb. I guess we're to infer one of her Dead Enders was the suicide bomber.
I seem to recall Dolly sending one of her people off with a bomb and telling her that Renegade would be found in the police morgue. But that was so much earlier in the episode than the actual explosion, with no followup in between, that I'm not surprised people missed the connection.