[The Saucepan Man]

That might change, of course, if I heard a loud noise and the earth shook, or an elderly person came and asked me to help them carry bags of soil. So, in this immediate possibility, things can affect my choices and cause me to change from a path that I might otherwise have taken

Yes, that's a very good point, and it's one that I was trying to get at when I talked of "fortuitous" events. The free will that the characters have is necessarily influenced, and therefore limited, by the circumstances in which they find themselves, the actions of others and random events. So, for example, if Fred decides (exercising his free will) to go from A to B, but there is an impassable mountain between these two points, he must go round it. He can choose to go round either one way or the other, but cannot choose to make the journey directly. His free choice is therefore limited by circumstances. Similarly, if Mary is told to do something by a person that she respects then, provided that she has no objections to the suggested course of action, then she is more likely to do it than not. Her free will has been influenced and therefore, in a sense, limited.

I see this as the way in which Eru's will influences events in Tolkien's world. For example, Eru (through the Valar) sent the Istari to Middle-earth to aid the free peoples in their struggle against Sauron. Gandalf, the most successful of the Istari, gains their trust so that, when he suggests a particular course of action, they are more likely to follow his advice. In this way he is able to "engineer" desired outcomes. So, by bringing together Bilbo with Thorin's company, he is able to engineer the defeat of Smaug, thus ridding Sauron of a potential ally and placing an obstacle in the way of a possible invasion of Eriador (and Rivendell) by Sauron using the route north of the Misty Mountains. Of course, he also relies to a large extent on providence (ie fortuitous events). As he explained to Frodo, Merry, Pippin and Gimli after the defeat of Sauron:

So it was that the Quest of Erebor set out. I do not suppose that when it started Thorin had any real hope of destroying Smaug. There was no hope. Yet it happened.

An example, of a fortuitous event that helped bring a successful conclusion to a "hopeless" Quest is the presence of the Thrush at the moment Bilbo tells the Dwarves of Smaug's weak spot. A manifestation of the will of Eru? Quite possibly. Similar events occur in LotR, in which the Quest seems similarly hopeless at the outset.

And, of course, a rather fortuitous event occurs as a result of his instigation of the Quest of Erebor that he certainly did not foresee. A certain Ring found itself in the hands of one Bilbo Baggins, rather than (as it no doubt hoped when it abandoned Gollum) the rather less safe hands of a Misty Mountains Goblin.

So, the free will of the protagonists is certainly influenced by the events that occur around them, and some of those events may be interpreted as the manifestation of the divine will of Eru (whether directly or via the guidance of His emissaries).

But it was not just Eru who influenced the free will of the people of Middle-earth. A few months back, there was a discussion about Turin and whether he exercised free will in the various unfortunate choices that he made. The argument was put that he did have free will, but simply exercised it badly, that he made bad judgements as a result of flaws in his character. I disagreed, arguing that, while he (and his kin) did have free will (and undeniably made bad choices on occasions), their free will was severely limited by the curse pronounced by Morgoth upon them. Most directly (and most obviously) via the intervention of Morgoth's emissary, Glaurung, whose hypnotic gaze and deceptions played a large part in bringing about their misfortunes. But I feeel that the curse also had a less direct influence on them so that, whatever choices they made, it would inevitably turn out bad for them. Their fate was predestined as a result of Morgoth's curse.

So, yes exterior influences did have a limiting effect on the free will of the protagonists. And both Eru and Morgoth were able to take advantage of this to bring about the outcomes that they desired. Of course, Eru's will being the stronger, it ultimately prevailed over that of Morgoth and Sauron, his successor.