Csharp! Your quilt is beautiful. I'm sure she will love it!
I'm getting ready to quilt one of my quilt tops, which has a lot of white in it (similar to yours). I'm going to do it on my Brothers machine. I'm terrified to mark the white parts. It is Kona 100% cotton (pre washed). I tried my purple marker on a spare piece and it disappeared. That's good news. However, I know from past FMQ that if you sew over some of these pens, the ink does wash out or disappear.
What did you mark your white fabric with and did you do it on a long arm or domestic machine? Thank you.