Hi Andrea- your quilt is looking great! I love the center of purples and white fabrics. Its very striking. (Question 2): I like the idea of adding a dark purple border between the center blocks and your applique blocks. That would frame it nicely and make the whole presentation of the quilt pop. (Question 1):I think it will look just fine with the added colors on applique blocks only.
Another idea for your applique could be to make some of the flowers smaller than others just for variety. (Question #3): I don't have any tips or tricks on preventing bleeding through applique pieces but have not had that problem in the few appliques I have done. I do use double stick fusible web called Steam-A-Seam 2 with my applique projects. It helps to secure the applique pieces well to the quilt.
Thanks for sharing the progress of your quilt. Its coming along so nicely!