The .5pb president confirmed that the movie is not a summary of the series and since it is a serious world line, it is not an adaptation of Darling either.
http://yaraon.blog109.fc2.com/blog-entry-3977.html
I doubt it's an original story. I'd say chances are high that it's an adaptation of Boukan no Rebellion, since that explores the side of the S;G story that didn't get enough development and focus.