[Tuor of Gondolin]

Very interesting post above. A few brief comments:
It seems to me clear that Middle-earth is not manichæist in philosophy. The primary cause of evil in Middle-earth (aside from its being rather necessary for a ripping good yarn) is free will. In some other forums some have argued for what seems to me to be a form of dualism in Ea, but I see JRRT accomodating free will with Ea being essentially "good". Two examples: Gandalf says somewhere in LOTR that he will not have wholly failed if anything fair survives (to me implying an eventually resurgence of good even if Sauron triumphed in the War of the Ring), and Iluvatar's comments to Melkor in the Ainulindale:
"And thou, Melkor, shall see that no theme may be played that hath not its uttermost source in me, 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."

This doesn't mean that rational creatures cannot do evil that negatively effects others and the world, but rather that (eventually) this evil will be redressed.

I hope this isn't too much off the topic, but the concept of free will in Middle-earth is interesting and rather ambiguous (especially as related to Turin and his family).