Took it in tonight to the second seamstress, she was just lovely and a doll to work with, knew exactly what the problem was and we came up with the solutions together. We'll be changing the back into a corset lace-up, hemming the dress by about 1/2 inch, and adding one 18 inch layer of crioline on the bottom to give it just a touch more fullness.
Now, she had nothing but amazing things to say about the quality of the workmanship done by my first seamstress, from how perfectly invisible the sides were done to the perfect "cupping" of the bust and how seamless the bustle was done. She doesn't actually have to "change" anything about what the first lady did with the modifications we're doing (meaning, so seam ripping and re-doing, it'll be left as is) because changing the back to a corset lace up will fix the waist and armpit muffin top issues.
I am so relieved! And, feeling pretty darn good about not asking for changes or money back, etc. What she did she did exceptionally well (minus the fitting me the way I want it to part), and she more than earned that money. Can't wait to update you all with the finished dress!