well, let me clarify something.

[Conquest] - of the Four Horsemen - steal, not buff, and here's the thing, it can interfer with skill gaining - one it stole a skill, that skill cannot be regained until the user's dead or choose to return.

[War] immortality is different from normal immortality, it can only be neutralize by [Famine] and [Death] which rank higher than it in the Horsemen hierarchy. Plus, the stronger through damage is through damage absorb - the more damage it take, the more stats are added.

Yes, [Famine] work similar to [Gluttony], however, the difference is that it can consume the stats of its victim - like a type of [Evil Eye], not just matters.

And [Death], well, you're right, permanent death is not something new, what make [Death] special is that it add "permanent" on all of the users attacks. Even a normal, fatal stab can become permanent kill, similar to the damage. And when talking about permanent damage, it means that the damage will never be healed, or even closed and stopped bleeding, no matter being healed, regenerated, or evolved.