Oh how lovely! The creative process is amazing and you are very good with it. I think you are wise to take some time away from this beautiful project.
First question - I think a wider margin would be nice and dark purple quilting would look wonderful and definitely tie the borders with the main body.
Second question - If time allows I think you would be happier with the dark purple quilting rather than the light on the bottom two corners.
Third question - Personally I like the corners the direction you have them. It reminds me of old fashioned picture frames and I think when it is all put together it would frame your quilt beautifully.
Enjoy your progress, so far it is amazing.