I could rephrase the Stunt as applying to people who share his convictions or are inimical to them. It's basically the same, effect-wise, but maybe it sounds more Conviction-y?
It still doesn't sit right with me, but better for sure.
Maybe.
Apart from piercing Devil Fruit defences, Armament Haki mostly just feels like a flavour for normal abilities.
I have Strength...because of Haki! I have Toughness...because of Haki! I have really high Weapons....because of Haki! And so on.
I guess it might be worth making the defence-piercing thing into a Power, but I'm not sure exactly how to do that. It's not absolute, so it doesn't just satisfy The Catch. There should be some kind of test, to see if your Haki is strong enough to trump a given defence. But how to represent that?
I'd say simply with Armor vs. Weapon rating based on Presence (or Conviction, although I think it fits less).
I don't think there ever was an instance where someone with Armament Haki active didn't manage to touch a Logia user, so I guess I would start with something like:
1. Haki downgrades the Toughness Power of a Devil Fruit user by one level (Immunity to Mythical, Mythical to Supernatural and so on).
Then I would probably treat it as Weapon rating:
2. Haki gives your attacks a Weapon rating equal to half your Presence skill value, rounded down.
Finally as Armor, with the caveat that you're not trying to dodge when you're using it that way (I can't remember anyone trying to dodge and still applying Haki to protect themselves):
3. You may decide to roll Endurance to defend yourself from a physical attack. If you do, Haki acts as Armor:X, where X is equal to half your Presence skill value, rounded down (maybe not halved? Or rounded up?).
It should probably come as a limited resource, though, both for balancing reasons and because it looks like you can "run out" of Armament Haki. But using Fate Points seems to be
too limiting. Stress boxes, maybe?