hi Mandie, your book on the Princess Elizabeth sounds so wonderful!! you would call that "historical fiction," as it would be based on fact, but, as you said, you would add "non-characters," etc. i am working on a children's book about the Princess Alice, Elizabeth's grandmother, that is based on the same concept.
i have 4-5 postcards of Elizabeth, i collect them, mostly the families of Hesse, Prussia, Spain, and Queen Victoria's family, esp. her children. i would be glad to scan them for you and share them that way.
Good luck!!!