1. Marcus was banking on grief and emotion as far as Kirk was concerned.
2. Honestly I have no idea. The torpedo thing has a lot of goofy elements to it. Scotty's whole "I CANNA LET EM DO EET CAPN CUZ I DUN NO DA FEEUELL..." Yeah, you do. You called it a Photon Torpedo. It's antimatter you mad-drunk bastard.
3. I don't recall Kirk telling Khan he was going to capture him? Are you referring to when Sulu told him to 'surrender or we'll blow you up' ? Because i'd wager that was done a narrow-beam transmission of some sort.