I would go with either a pink binding like your inner sashing or the green like you've got in the squares. I like the pink idea the best though. If you want to make your binding wider, choose whatever seam allowance you like. Then, when you get to the corners, stop sewing that distance from the corner. ie if you use a 1/2 inch seam allowance, stop sewing 1/2 inch from the corner and do your flipping to turn the corner. That way, you should be able to turn the corner with no problem. Practise with some scraps of material the same thickness as your quilt till you get the hang of it. Your quilt is awesome. Love your colour choices.