[Raynor]

Originally Posted by Aiwendil

I feel I must confess up front that the Ainulindale is one of my favorite pieces of writing, by Tolkien or by anyone else.

I think you spoke for most (all) of Barrowdown

Originally Posted by Aiwendil

One thing that strikes me is the crucial role that the Ainur play in the creation, which doesn’t seem to be mirrored by anything in the Judeo-Christian story.

Then again, that is downplayed by the fact that all themes originate in Iluvatar: "and thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me"

Originally Posted by Aiwendil

Though I wonder whether to see some connection here is to mistakenly read elements of Tolkien’s real life and religion into his sub-creation.

In his view, myth-making is to return to our initial state, before the Fall, and it reflects God's Truth. Imo, these are all of Christian nature to him.

Originally Posted by Aiwendil

We are never told what exactly this is, though it appears to be some kind of creative force, and it is said to be “with Iluvatar”.

Clyde S. Kilby notes in his book "Tolkien as Christian Writer" that:

Professor Tolkien talked to me at some length about the use of the word "holy" in The Silmarillion. Very specifically he told me that the "Secret Fire sent to burn at the heart of the world" in the beginning was the Holy Spirit

Originally Posted by Aiwendil

For instance, is all of creation embodied in the Music?

.
No:

Originally Posted by Ainulindale

Yet some things there are that they cannot see, neither alone nor taking counsel together; for to none but himself has Iluvatar revealed all that he has in store, and in every age there come forth things that are new and have no foretelling, for they do not proceed from the past.

Originally Posted by Aiwendil

If so, what implications does that raise for free will in Arda?

Free will operates within certain limits:

Originally Posted by Letter #153

Free Will is derivative, and is only operative within provided circumstances; but in order that it may exist, it is necessary that the Author should guarantee it, whatever betides : sc. when it is 'against His Will', as we say, at any rate as it appears on a finite view. He does not stop or make 'unreal' sinful acts and their consequences.