Here's a good idea for you:
Maybe intervene after the betrayal, let Naofumi do his thing until he gets Filo, then become amigos?
Not a bad idea, maybe have Itsuka be the villain. Because the sword hero would go and kill the dragon, which would lead to Naofumi getting the Wrath Shield. Basically, everything would go as the plot intended, but this time you actually have a chance with Filo.