Oh Pam, another beauty! Hmmmm...I'm thinking. I know you can do the feathers and you know thats what it is asking for. Just doodle them alot and your brain will get trained for them. They really aren't all that hard with a little practice. As far as for the diamond areas...maybe your original ideas of the s curves would work well. I'm hand quilting my Jane right now and she used a lot of melon type shapes in the blocks that have lots of diamonds in them. It seems to work well with the geometric diamond shape.
Whatever you choose will be fabulous though, I know!