I took it home and gave it a good inspection. I discovered that the belt was several inches too small; which made me realize that this dress is probably an x-small and meant to be oversized.
I unpicked the release tucks in the shoulder area and found that I would have enough fabric to shape the body into small extended cap sleeves. I carefully cut off the turtleneck, being aware of the button placement, and changed it to a scoop neck. I finished the neck edge with single fold bias tape. I tried on the dress and pinned the placement for the new side seam and extended cap. After several try-ons I got the fit just right. The finished the sleeve openings with a narrow hem.
The belt was not so easy to fix. I cut off the buckle and grommets to make a symmetrical piece. I tried doing a whole thing with elastic in the back but it didn't work out, so I ended up using the sleeve fabric to make ties and carefully sewed them to the front belt piece. I say carefully because on the first try I was not careful, and my machine fell apart trying to go through the plastic belt backing. Whoops. After using the World's Smallest Screwdriver to put my machine back together, I removed about 1/2" of the backing on each side and sewed through just the fabric.
And now for the final dress....