It's very pretty! I really like the colors and fabrics you used.
I started quilting in February of this year and have learned SO much, from this group and online!
I too use my bed for layout, since it's the only place I have to do so. I use the 505 spray often, but have had no problems with just pinning either. Once the sandwich is laid out, I set up a folding table at the foot of the bed, then pull one end of the sandwich up onto the table to smooth, stretch and pin it together. (or spray and smooth) I smooth out the backing fabric, then spray the batting, or smooth it if pinning, then spray the batting top and smooth the quilt top into place. If I'm just pinning, I reach under and place my first pin in the center, then work in a "circle" around that pin, pulling the backing fabric tight, then the top fabric tight before pinning in each place.
The fabric DOES stretch as you quilt, so you have to keep pulling it tight, without actually stretching it, of course, as you go. I assume this is the reason for cutting your backing and batting larger than the quilt top. After I quilt about the first 1/4 of the quilt, in the center first, I then lay it flat and make sure the remaining areas are still smooth before moving on.
I think the "looseness of the fabric" is because it's not quilted, not attached to the batting or backing fabric. Have you washed it since finishing it? I find that seems to tighten the fabric back up, too.
A walking foot makes all the difference in how the layers feed through the machine.