I LLLOOOVVVEEE your shirt. I love the colors and the details you've added. Most of all I love the little cuff you added with the contrasting fabric. You are very right about the buttons, they do leave much to be desired. Your pearl idea would be wonderful. It would be whimsical and unexpected for such a shirt but feminine and classic at the same time. I know you'll come up with something awesome...you always do.
As for replacing buttons, I do it all the time. I have bought really cheap blazers from Walmart and replaced the buttons and I have been known to buy things at the Goodwill just for the buttons. (yes I have a problem!)
Now lets address the collar. Okay hear me out here. What about making a casing and putting like a drawstring tie in it, just for detail. Or I don't know how much you have but what about braiding the fabric and hand stitching it around your collar with your contrasting fabric...I'm super into braiding fabric right now.
Just a couple of ideas. Can't wait to see what you come up with. I am going to work on a skirt for a 7 year old tomorrow. SUPER EASY with no pattern. I'll take more pictures than usual and show exactly how to do it. You can do it for any size. Check with me tomorrow.