He defied Eru only in the sense that he went against the grain of the music, but since Eru is omniscient of the whole musical piece, even defiance of direct wishes from Eru is still part of the overall plan. And sometimes a musical score needs a bit of dissonance here and there.

Even the idea that Eru stepped in at the last moment to help the Ring off the cliff works, because that's akin to silently tuning an instrument before the next movement. Gandalf being sent back upgraded the same.