1) You're right, Arthur is attack buffs. My bad. I'm thinking of Joan of Arc.
2) These scenarios seem unrealistic. Unless you're willing to drop mad cash, and max out a team of SSR hime/SSR weapons, you're not gonna have a good time trying to damage down a team of 3 tanky trash mobs before they overdrive you. I'm at 20k total team damage right now and there are still maps where I can't clear trash mobs efficiently in 1-2 turns, even with the stacking damage bonuses from double apocalypse + a fire team. And we haven't even gotten the really difficult maps yet.
Same with the tougher bosses. You're better off running someone with a mitigation skill to block their big overdrive spike and multiple healers to keep up your team than to rely on doing ten million damage every single cycle with skills to take them from 0 to rage and then rage to stun instantly every time.
3) It's true that healing is capped, but damage is also capped by money - unless you're willing to wallet warrior the shit out of this game, half your team is probably gonna be R/SR hime without ideal skills or the perfect element set and that's gonna put a FAR greater cap on your damage potential than your heroic soul choice. Yeah, its true that with enough damage you could probably bull through literally any fight, but if you're looking for the most cost effectiveness and early game value I wouldn't go any of the pure offensive heroic souls at all. Once you get to level 50 and have a choice of a second one, that's a different story.