I may be reading too much into this, but what if IC did want Ember to be the Unraveller, and deliberately made the prophecy ambiguous enough so that people would question it?
But that said: I'm still on the fence as to which one of them is actually the Unraveller, if not both. However, I'm going to try and go with the theory that they're both the Unraveller, just to get it out in the open.
(I don't have a copy of the books on me, so forgive any mistakes, okay?)
Half blind yet seeing all: Ember drifts off into her own world, and relies on Glynn to handle things for her. In Darkfall it's mentioned that Glynn handles all the transactions of Ember's music because Ember doesn't care what happens to it. So maybe Ember's insularity is a type of blindness that Glynn counters?
Marked by visioning, but without Darkfall mark: They both segue, and neither of them is properly marked by Darkfall. Also, neither Glynn, nor Ember fit the description of a soulweaver (passive and serene etc), which may be a mark in itself. There may also be other marks that come during the initiation on Darkfall, ones that no one else sees. After all, how is a soulweaver to see another soulweaver's silverblindedness?
Who lives, yet sings the deathsong: Well, here I think that the poster who mentioned the fact that everyone around Glynn seems to die deserves applause. That was a great idea! And Ember's part of this is obvious.
Who is born, yet not of the Song of Making: I may be wrong here, but didn't the Song only create Keltor? If so, neither Ember nor Glynn are actually of the Song, despite the fact that Ember is so gifted with music. That in itself, may fulfill another part of the prophecy, where Ember's talent for music dulls Glynn's "blindness".
Who is gifted from the great water: Both of them arrive in Keltor through the sea, as do any strangers to Keltor.
Who is crowned in bright flame: I know about the whole red flame versus yellow flame thing, but I reckon that it takes both colours to make a proper flame. Remember the drawings we did in kindergarten where we coloured the outside of the flame red, and the inside yellow?

![Bouncy! [bounce]](./images/smilies/jump2.gif)
But we'll just have to wait for Darkbane when everything is resolved. ![Crying or Very sad [cry]](./images/smilies/weeping.gif)