#1: yellow and purple are complementary colors. When next to each other they command attention. The center is beautiful. Having yellow and/or orange in only the exterior blocks will draw the eye to the outside. I think the lightest colored squares could be very pale yellow print and yellow corner stones in the sashing.
#2: an instructor (I think it was Ruth McDowel) says, "make visual decisions visually." Put some dark purple fabric where you're thinking of putting it. Then decide.
#3: to avoid bleeding I can think of 2 solutions. One is is to cut out the background fabric behind the applique. The second is to first face the applique with a light weight interlining of white.
Happy Quilting