I basted the layers together along the seam line at the top of the corselette, and turned the center back seams inside, but left the seam open for placement of the hook and eye closures.
So, how did the fitting go? Very, very well! She was able to get into the corselette all by herself (but missed a few hooks and eyes in the middle) and it only needs one more adjustment. The front is a little too wide, so I need to take the center front seam in about 1/2" (for a total reduction of 1"). I threw the gown muslin back on her, and pinned and marked it up accordingly. We added 3/4" to the top bodice edge for modesty, so I'll have to add that to the gown bodice also.