It's going to be hard to get just one clear cut answer to your questions. As I'm sure you noticed by now. All is not black and white in quilting. Some people always prewash. Some never do. Some never do until a red bleeds, then they always do.
I don't prewash, as of yet. I also present all quilts unwashed. In my eyes, when I hand them off, they're good. I know they will get wrinkly after being washed and tell the new owner to wash in cold water and it's up to them how they want to dry the quilt.
If it's your quilt, you can add batiks in your scrappy quilt if you want to. I'd recommend laying your scraps out on a table or a larger piece of fleece [(that's been stapled to a large piece of cardboard). This fleece piece can lean up next to a wall and all cotton pieces will magically stick and not fall down]. See how the batiks look, if they blend or stand out. It's your quilt. Do what you want to and learn from it. It was all a gift, after all. When you are done show the gift giver what you've made! Good luck and welcome to the wonderful world of quilting!!!