Isn't a person's favorite food and whatnot the very definition of 'trivia'?
I do agree that small details help to flesh out a biography, which is why I love Pope-Hennessey's bio on Queen Mary so much. He didn't list her favorite foods but there was such a color and flavor to the book rather than a dry recitation of facts. Is that what you're getting at?
As I said, check out Gould-Lee's Empress Frederick Writes to Sophie. For now, at least, that's as much information as has been put out about young Margaret's early loves, engagement and eventual marriage.