So, I decided to press on with the cording and see what happened. Inserting the gores seems to have eased the problem of the puckering at the bottom of the underarm cording. When I stretch the corset it dissapears completely - I think I'm going to get away with it! :) The big question now is whether it will fit well enough to put the fabric under enough tension to pull it straight.
I also made the button tabs for the center front. They are a single layer of coutil sandwhiched between two layers of saitn, and I love them already! I've stitched buttonholes but don't plan on cutting them, instead iI will stitch the buttons on through both the tab and the corset, but I wanted to have the buttonholes there to peak out under the buttons. I don't have suitable buttons yet, the ones on the original are fabric covered and very flat, possibly just a metal ring with fabric stretched over? I will have to experiment to find the right technique.
The cording, by the way, is starting to drive me nuts. mostly because my threading tool is shorter than the longest channels, I will have to do something about this before I tackle the other side! But on the plus side, I'm clearly still in love with this design, because I keep coming back to it when I planned to start work on something else!