I'm not much help for decisions, Suzslo -- but I like your contrast between the aqua and cornerstones. I've been muttering at mine... Personally, I think you do need something on the outside and rather like that thing you've got going on the top.
I'm continuing to make progress -- it's amazing what happens when you (a) have fabric, and (b) actually sit down and sew.
I have my setting triangle units made but need to layout the top in progress and set/number them correctly. I'll be trimming them and stay stitching that raggedy edge before assembly into rows. I am putting on the sashings as an L shape so it works slightly different than the way Bonnie shows -- it works for me and I admit to being an old dog some times.