[Tuor of Gondolin]

To briefly interject, it's seemed to me that (partly from the above quote of Iluvatar)
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Then Iluvatar spoke, and he said : '...And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me[emphasis mine], nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.'
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that Iluvatar controls the overall plan and fate of Ea and it's
inhabitants, but that within this plan a role for free will is allowed. Morgoth and Sauron revolt and have a considerable effect, but eventually are interceded with
(such as at the end of the First Age allowing the Battle of the Valar, permitting the Faithful to survive Numenor, and sending the Istari). The free peoples can, and may fail, and that would have consequences, but I'd suggest that eventually Eru would intercede to put Middle-earth "back on track" even after Sauron got the ring.
Several quotes by Gandalf (I can't look them up now) seem contradictory
but may not be.
One of them says something like, Sauron with the Ring would dominate earth as far as can be seen , but the other that he (Gandalf) would not wholly fail if anything beautiful (a flower?) survived. Which suggests, to me, that a backup plan would kick in if the quest failed (though that plan might take many years to be effective (for example, perhaps the Blue Wizards had not wholly been in vain and some of the seeds they sowed in the East would lead to revolts against Sauron?).