So... The mockup. I was originally going for a 26" waist, which is two inches smaller than my actual waist measurement. Somehow, I ended up with a 32" waist. I'm really bad at math. Basic arithmetic I'm fine with, but fractions, percentages, etc., I'm lost. I really do think I'm math disabled.
I also think my mock-up doesn't look quite right. I sent a picture of it to Jo and she said it looked fine.
Next mock-up I'm definitely using a patterned fabric. I got confused on how to sew the pieces together, even though they were notched, and I think if I had a fabric that had a clear "right" side and "wrong" side, it would be easier to figure out.
Here's what Jo suggested to fix the mock-up so I don't have to start over:
So it's 32 and you need 26 inches, that's 6 inches that needs to be removed. This is what I'd do, I'd take 1/2 inch out of each panel by folding each panel piece in half on the grain line and sewing a seam 1/4 inch from the fold. Boom, 5 inches gone. Then I'd fold the corset at the center front seam and sew another seam 1/4 from the original. There's another 1/2 inch gone. And finally, I'd bump the center back edge in 1/4. That should take care of the last 1/2 inch. Remove those those 1/2 inches in each panel, and the 1/4 on the center front and back. You can reconnect the top and bottom edges after fitting because you may need to add or subtract at the underbust or hips anyway and will have to check those edges anyway.
You're right, once you take a section out of the center of each panel top and bottom edges will no longer match perfectly, but this mock-up is about getting the fit right and uneven edges won't really affect that. You will still be able to adjust the fit by either pinning or letting out the seam allowances, and you'll still be able to gauge if the length works for you.
I'm going to use a colored thread to take in the panel pieces so I don't get confused and think the seam allowances are the trimmed pieces and vice versa.