YES, the sword may just have a cool design, I think I said that too. Nobody knows Kishi's plan, all we can do is make intuitive guesses. However, Kishi is a smart man, and uysing a double helix design for design alone amounts to cruelly trolling the fans, do you agree?
Yes, the will to never give up is important, but what really made the Bijuus believe in Naruto is the way he was the first human to Care about them, and not bear ill will toward them, To not resent being made a Jinchuuriki, to accept them as living beings and even appologizing to them.
Just to be clear and certain, Naruto has not defeated the Juubi, Both He and Sasuke together, defeated Obito.