Not sure how this fits into the Tolkien world, but I'll throw this in. Presumably the whole universe is governed by 'laws.' Planets, stars and solar systems just don't run around willy-nilly; their behavior is governed by laws like gravity etc. There were all of those laws that Newton 'passed' that proscribed what an object (whether at rest or in motion) can or cannot do.
Organisms on this big ball of rock on which we live are also governed by laws. One might say that it's all in the genes, and that when you make a choice, well, you are acting not as a free agent but as a product of what is encoded in your DNA in response to something in your immediate environment.
I like reading, especially Tolkien. My biological father and eldest brother are also avid readers (yet not Tolkien fans), and as I was not raised by said parent...it could be genetic. I was exposed to Tolkien via the school library whereas my father and brother weren't (or most likely they too would have read Tolkien).
Then there are other laws that govern one's behavior. Surely you felt the urge now and again to scream, "PJ is God!" while swinging a homemade replica of Gandalf's staff around in the air in a crowded movie theater while watching LOTR, but there are social mores and also legal issues to consider that (hopefully) governed and possibly restrained your behavior.
I think that Isaac Asimov wrote in the
Foundation trilogy scifi novels that societies' actions could be predicted and therefore were deterministic. All that you needed were large enough numbers and you could use math/statistics to determine what people (not individuals) would do. My guess is that once you gather some priors on persons, you then could determine what 'choice' they would make. Isn't this what marketing firms do when they gather information?
At the quantum level, however, the universe isn't deterministic but probabilistic, meaning that you can't know exactly what will occur but can make some really good guesses.
So, whether it's Newtonian physical laws, my genes, my environment or simply the most probable action, I just had to post.
Anyway, assuming the Middle Earth world is similar to what I have above, then I would say that chances are if Frodo had not taken the Ring, then maybe we would have seen "Fatty Lives" spray-painted in American subways. Maybe if Eowyn would have been lost in the initial charge of the Rohirrim, we might have had a site praising Ioreth or her sister. If Frodo or Eowyn would have fallen, then either someone would have jumped into their places or we would not have had the chance to read LOTR as Sauron would control all printing presses and so we'd be stuck reading Robert Jordan of "Wheel of Time"...ahhh...fame.
Eru was merciful.