The Treason of Isengard: Eru's Plan?

[HerenIstarion]

Well, burra, let's see what we can do with your attitude:

If I take the road that heads east from here I will not end up in California, unless I turn onto the westward road

But if you walk East long enough you'll land in California, I surmise?

Similarly, if Feanor decided to be a peaceful fellow he would not have been killed by balrogs.

Imagine a scenario - Fëanor says yeah, does not revolt, all elves - Vanyar, Noldor and Teleri remain happy family, there is no Exile, no Prophecy of Mandos and no Siege of Angband and no War of the Jewels. In due time, however, Valar make a council, and there is War of Wrath, just Noldor form the part of the Host of the West this time, they are the saviours rather than remnants of the people to be saved. In a mighty assault Morgoth is defeated, but as his last effort at defending his stronghold, the band of Balrogs issues from the Iron Gate to challange the foremost party of the assailants lead by Fëanor. Ultimately, Balrogs are swept, but Fëanor is killed.

But all of the above is exercise in free modeling, just so. As seen from my previous, my own position is as follows:

each derivation of choice does alter the consequent chain of events

That there would be Fëanor in due course of events, that he'll have certain temper and talents and moods is all in the Music. What exactly would Fëanor do with his talent and temper and moods is up to Fëanor himself