Progress update:
I finished the corselette:
I added an extra layer of firm cotton to the two layers of lawn. I experimented with a petersham waist stay but I could not get it so that it was both tight enough to feel like it was holding everything up and loose enough that I could sit down and eat! Instead I sewed the petersham ribbon around the waist of the corselette and added an elastic stay for extra security. A bias strip around the top matches the fabric of the underbodice. You can see the hanging loops too, which are secured within the seam allowance at the waist. Once the dress in finished I will add some crocheted thread loops to hold them in place at the top edge.
I then had a complete disaster with my bodice
In spite of what it looks like in the picture, this actually fits. The main problem, I think, was the way I applied the fabric and underlining together (i.e. really badly!), causing the horrendous wrinkly mess you can see above.
So I started again. I recovered the back panels but had to recut the front panels. Luckily they don't use much fabric.
I added an extra layer (calico at the front, and lawn at the back) with a few more strips of boning (originally I planned to only have boning in the corselette, but it was definitely needed in the underbodice too). It's only tacked together so far (hence the weird looking extra visible unpressed seams) but I am much much happier. Some of the boning is visible from the outside, so I plan to slip in some strips of cotton flannel between the layers where needed.