I think I have solved the bridesmaids dress dilemma. I called a seamstress (someone who had great reviews online) and asked her opinion. She thought the size 18 would be overkill, and she told me to go to David's Bridal and call her to let her know how many extra inches of fabric were around my waist (or what used to be my waist) and my hips. So I went to David's Bridal and even borrowed their little "baby bump" to try on underneath the dresses. I wrote down the extra fabric inches on the size 14 and 16 and called the seamstress. She said to buy the 14. She's going to try to sew extra panels of another fabric (closely matching the color of the original dress) to the liner underneath. She sounds like she knows what she's doing, so I'm hopeful that it will turn out ok.
Problem solved (hopefully!).