You have a great design and I love the colors. I think with a little experimenting you'll find answers to your questions. In addition to the many great suggestions already posted, try these too:
1. Vary the color placement in each pane - so each pane is unique.
2. Try using the yellow in the smallest pieces in the panes. Yellow is light and bright and by using it in smaller amounts, it won't stand out as much, yet will add sparkle. Example: top left pane.
3. Try a darker or more subtle color for the cornerstones so attention is directed to the panes.
6. My first thought about binding was the purple. I usually take a darker color from the center and use it for the binding. However, I really like the ideas previously posted about using the yellow in a small border or as a binding. I would wait until the center has been completed, then audition borders and binding. Sometimes the quilt asks for something other that what we had originally planned.
Have fun with the design phase and try lots of alternatives. Please post your final design as well as your quilt.