The light aqua fabric you are thinking about using for a sashing - the design seems "frail" compared to the "strong" prints in the blocks - IMO
Working with a variety of blocks like you have has always been extremely challenging for me.
I actually liked the look of them laid out on the first picture with a cream or light background so the blocks would stand out. I like to use cornerstones when using sashing - I think it is easier to get things lined up when using them.