You will have to remove the skirt from the bodice and straighten the grain. Otherwise, the dress will continue to twist on her. It is possible that the fabric was off-grain to begin with and the store brand just ignored it. This is often found in garments from 3rd world countries or China. To straighten the grain, you must open up the side seams and the back center seam. As the fabric relaxes, measure the grainline and recut the skirt. You should do this for the lace overlay too. Since you mentioned that you have excess fabric because of the wrong sizing, it should not be difficult to do. Hope this helps. Don't hesitate to ask more questions, I have done bridal wear as well as other garments and I constantly do alterations for others. To me it is much easier to make a dress than to alter one that doesn't fit right. Here are 2 pics of wedding dresses that I have done in the past 2 years. The first dress is one that I made for a friend and the second dress is one that I designed and created a pattern for my cousin's wedding last fall.
Brighten each corner where you are ... smile!