It's a classic fantasy trope.
No it isn't. The magic fuels his strength and endurance. The skill is 100% Elrics.
No it is not. The sword specifically moves his arms and even kills and drinks people on its own and against his will who he does not want to attack.
I read the books. It seems that you've forgotten the potions that restore his strength and vitality. He did not need Stormbringer. Stormbringer just managed to function as those potions when it drained souls.
No I haven't, but it seems you forgot that he ran out of those potions in the Desert when he left Melibone and the sword had to save him.
Also it is explicitly states in multiple fights he is in that he could not win without the sword and there are fights beyond that which he can't win without using Sorcery to call on Chaos.
In 5e the paladin is a caster the way a catfish is a cat. I mean, kinda sorta, but not really. A paladin is a smiter.
If your argument is for a more powerful Paladin that can do supernatural things then we are not in disagreement. But that is far different than saying a fighter or Barbarian should be able to.
Not fanatasy.Zoro from One Piece.