Ridiculously fun! And really the best way to raise chicks. Are you not glad you hatched them out! What fun. I love this hobby.
I don't know if it is still going, but a 50% hatch is (or was) the last time I checked the average. Yes, some get more, and some get less, and you can go a long stretch...
I think she is succeeding, and I would leave her be. However 15 eggs are a lot to cover, but it they are developing, leave them be. They definitely won't make it if you break her. She knows more about being a chicken and raising babies than we do.
What has happened when I moved a broody, is...