Ok, this is gonna be a long post lol.
First, here are my sketches of what I think is going on in the Atonement Dress.
It's a blousy top, gathered (with big gathers) at the waist into a band, which is then covered by the belt. In this picture, http://www.telegraph.co.uk/fashion/graphics/2007/09/02/st_keiraknightley.jpg
it is also looks cowled at the neck, meaning the it's cut wider than it looks, and it drapes like that when the shoulders are pulled in. I have no idea how it stays on her instead of just slipping off her shoulders. Maybe if the straps are tight enough down the back? Also, the shirt appears to "twist into" the spaghetti straps at the top, front. There is also a laser-cut out pattern around the neck and sides, with something resembling a bow at the V-neck point.
The skirt is fitted at the top, but very full at the bottom, yet has no visible seams (argh). Most likely it is a circle skirt. I can't even see side seams, but by looking at the pictures closely, it looks like they might be folded in under the wrap, so that they will fall within the vertical ruffled bits, and be hidden. Then, the back is split and there is more fabric inserted for added fullness. The dress is longer in the back than the front--it has a train. Probably the easiest way to do this is to make only the added bit get longer, though in the original, there is a gradual slope back from the front. (You can see this in the pictures, where the dress is off the ground in the front, you can see her shoes, but by the side, it is already sweeping the ground.
Edit to add: One article says there is a high slit up the front of the skirt, if that's the case, the front is probably 3 pieces, a top, and then two side bottom pieces (like in the Vogue pattern), the connection of which is hidden by the wrap.
The belt, unlike my drawings (just figured this out), comes around in back and is then tucked under the wrap. Hence, no seam, and the fluid, completely wrapped look of the back.
The wrapped part, I think, is a loop, wrapped through itself on the back, with the seam under the part where it crosses. The two "flat parts" of the loop are then gathered in the front by a separate tie. (The dress does not tie in front as in the replica). This "tie" appears to be a piece of the fabric that has been folded at least twice, so that it is very narrow where it passes over the wrapped fabric, and then widens significantly, as it unfolds down the dress. The entire wrapped piece would then be invisibly hand-sewn to the gown to keep it in place.
So...I guess my question now would be, how close of a replica are you interested in? Because that vogue pattern would need some serious changes! I think any attempt to do a close replica would need a lot of draping to get it down right.
I think for a quick, similar gown, I might start with the Vogue pattern, but eliminate the gathering detail on the back, to be replaced with the wrap. You might also need to move the seams in the front up, so they will be covered. And I would add much more fullness at the bottom (slash and spread the pattern). Add the belt and Wrap, and it would be pretty good, IMO.
As for the Emerald fabric, they had to dye their own for the movie