Oh, I feel your pain! I've struggled with the double wedding ring pattern and it isn't half so complicated as yours. Something that occurred to me when sewing some non-quilt items, though, is that I've set sleeves into garments and faced necklines for years. Curved seams! I agree with bearisgray that hand sewing might be your best bet. What about some hand basting and then machine sewing? I also wonder about running a basting thread along the convex side of the seam and slightly gathering as you sew; not enough to cause puckers but enough to ease in fullness.