[Elladan and Elrohir]

The breaking was kinda a moot point because even as Feanor spoke, his father was being slain and the Silmarilli stolen by Morgoth. Obviously, though, Feanor's decision had a great impact on his next big decision...

I feel certain that if the Noldor had stayed home instead of chasing Morgoth and the Silmarils, the Sindar and the Edain would not have thus been doomed. I don't believe the Valar would neglect them and leave them to the tyranny and oppression of the Great Enemy, and even if they did, Eru Iluvatar certainly would not.

This all is entwined with the Great Music. In Eru's plan, even evil would ultimately work for good. We can be sure that it was not his plan for the Noldor to rebel against the Valar and go back to Middle-earth, but he still used it for good, as they began the War of the Great Jewels against Morgoth, which preserved the lives of the Sindar and the Edain. We can be equally certain that if the Noldor had not left Valinor, everything would still have worked out all right (as all right as it can be, anyway) in the end.

As a Catholic, Tolkien was a strong believer in the sovereignty of God, and this definitely carries over in The Silmarillion. His God is a flexible one, whose plans are ultimately disrupted by the shortcomings of his creatures. This is the context in which we must view the rebellion of the Noldor.