A little bit of digging has turned up new references! It turns out that the dress was actually displayed and photographed, so we have some excellent high-res shots of it on a mannequin! These great images give us so much more information.
I was really surprised at the fabric of the underskirt. It's patterned! It's a rose patterned brocade in the same color as the overskirt and bodice. I was also surprised at how much lighter the color of the dress is off-screen. It appears to be a very deep blue in the film, but photographed in a studio, the color lightens significantly. These photos also clear tell us the type of fabric, a lovely duchess satin, which is different from the taffeta most costumers have used to recreate it in the past.
In these photos you can clearly see the scalloped edges on the underskirt and the pleated ruffles. It's also clear that they're narrowly pleated, at about a 1/2 inch, and not fluted. There's also a peek of a black ruffle underneath the blue. There's like a black ruffle on the inside of the skirt to help give the skirt volume. The black ruffle isn't pleated, just gathered. The overskirt appears to be lined with black fabric.
On the bodice, we can see that there's a line of piping at the bottom edge of the front. It ends where the peplum begins, and then the peplum is edged with a pleated ruffle. The round trim on the peplum appears to be small velvet disks with beaded rosettes in the center.
There is also piping at the neckline edge and around the armholes.
I love stumbling upon new images of gowns I thought I knew. These new pictures definitely expand my knowledge of the construction of this gown! I can't wait to dive in and make my own.