Wow, you are really ramping up the emotional load! - with such vivid, dramatic writing, it wrings the reader's heart. I so like the description of Gandalf, weary in his flesh, conscious both that he is more than mortal, and that he is "just Gandalf, an old man who was tired in his bones." The moment when he, like Legolas, is caught by the scent of the sea and wants to go home is really moving. We so rarely see Gandalf vulnerable, and yet he must have felt so much strain at this stage of the story (even in canon!)
As ever, your descriptions of the Music and the different threads of the Song are lyrical and lovely. I do like the idea of Radagast being able easily to read and gently to manipulate the Song, versus Gandalf the wizard of words.
Oh, and poor, poor Elladan and Elrohir. Both of them. If you do manage to bring them both positively out of this fic, I will take my hat off to you, because they're a long, long way from happy-endings just now...