It sounds like a very cool plan you have. Regarding your backing for the tail fin, couldn't you just cut your backing way oversized to accomodate the overhanging tail, and do the same for the batting. Quilt as desired and then trim away all that excess once quilting is completed. Then work with a bias binding to accomodate all those curves and odd angles when binding.
For painting, well I have never done fabric painting but in painting hard surfaces a dry brush technique works quite well for subtle shading. Or stencil brushes and shiva paint sticks might be the way to go. Because you work a lot of the paint off of the brush before applying it to your surface you slowly build up to the tone/shading you want a little at a time. I would recommend getting Irena Bluhm's book "Quilts of a different color" for ideas/guidance.