Brenden, that's all correct. If you're worried about the lofting (and, also, splitting the down tube into two), you need to either do the two sides separately, or do the work at an existing seam.
Another possible approach, which will look ugly, is too simply fold the excess fabric out of way, and sew those pinched areas up. No need for a seam treatment, because you're not cuttin anything out, just making a dart by sewing it up. (If there's a lot of fabric, you can roll the folded out up, and sew the roll together.) Do that on both sides, and you're set.