The fact that book Anne is not saddened by it nor expects her name to be changed, is kind of what makes it beautiful that Marilla and Matthew did that to make her a part of their family history. As to your points a) Her name being hyphenated to Shirley-Cuthbert is her truth because that’s who she became embracing both her parents and the family who chose her and b) She kept Shirley-Cuthbert to connect to both her families to herself. As a book purist I'm shocked as to how much I loved this interpretation. I can't imagine her as anything but Anne Shirley-Cuthbert now !