That is my point. There is no point in speculating what Feanor might have done if Melkor wasn't there becuase Fate had already planned the course of his life for him. Thus Mandos knew that Feanor's life would be short and that 'He would come to him soon'.
Actually, I agree with Glorfindel that such problems are worthy of discussion because usually things that are 'fated' can have only one outcome, although many paths can lead to that outcome. Therefore, whatever decrees of "fate" or "doom" that Eru set down in the Music or earlier in his own mind should be able to have been accomplished regardless of how it came about.
For example, Eru probably meant for Feanor to lead the Noldor back to Middle-Earth. Therefore, I think that such a thing would've happened, regardless of how. I mean, it was Morgoth's lies that led to the theft of the Silmarils and the murder of Finwe in the story; but it could have been due to something else, perhaps Feanor's unhappiness at his step-mom and half-brothers. It does say in the Silmarillion that Feanor was not overly happy when his father remarried. And that was BEFORE Melkor spread his malicious lies. The point I'm trying to make is that Eru set down a root of jealousy and arrogance in Feanor, and that root would have come to germinate, bud, and bear bitter fruit in the end regardless of which factor sparked it. And the factors that sparked it could have been many. It could've been Morgoth; it could have been a nastier son of his. It could have been a passing jest. It could have been simply growing self-obssession on his own part... But no matter what it was, it would lead to the same result. Feanor leads Noldor out of paradise and into the real world.
So it is actually quite fun to talk about the other possiblities because it can prove that what actually happened was inevitable. And in the end, of course, you will find out that although there are many ways of a certain Fate working out for one particular character, there is only ONE way that the fates of ALL the characters can work out together. Which is why you can call my last paragraph useless, but I just wanted everyone to see the process of thinking... that's all. It's from the process that we learn and understand, anyways...