Yes. Usually there is a fitted lining that’s boned. The sheer outer fabric is mounted over it. If an Edwardian dress is sheer completely, it was worn over a boned under bodice.

If your waistline is fluctuates in height it seems like your corset is not sitting in the same spot each time you put it on. Can’t think of what else would change that. Perhaps simply use the corset to smooth out your silhouette rather than for cinching. In other words, keep your corset a bit loose around you so functionally it’s there to provide a stable base for your bodice to fit around. Then you’ll have a bit of room to ease “in and out of shape.” I think your “approximate fit” might be best. Allow wide seam allowances at the side seams for quick adjustments.

Oh, and make sure whenever you put on your corset you give yourself a good 15 minutes or more to settle into it. Don’t try to do a fitting or put on a fitted bodice until your body has adjusted to the restriction. Afterwards, cinch in the extra the develops… because it will.

Building in ease helps. I had it fitted exactly right yesterday. It looked good and felt good. Today the waist is too high and the front won’t close because I’m an inch larger. I don’t think I’m going to be able to do more than an approximate fit. Have you ever had a student with this problem? How was it solved?