But from everything I've heard the movie is a watered down and crappier version of the book, so I would strongly recommend reading it before you jump to conclusions.
That said, you're right in a sense. Ender is a prodigy, he's great, he overcomes all obstacles, etc. In that sense you can call him a Mary Sue. However, I don't agree with you about calling Ender blameless. I never saw it that way in the books. In fact, the issue of blame wasn't even something that crept into my mind. The story is about Ender's dealings with the environment, and his philosophy of minimizing losses through overwhelming aggression. It is left up to the reader to decide whether that is really a justification, or if Ender is a brilliant psychopath, but it is an important question, one with real life and historical applications.
In a way, I suppose it is a childish power fantasy. Of being Ender, the guy everyone underestimates but who then rises to the top, squashes his enemies, etc. And to be perfectly honest, I'm not ashamed to admit that I enjoyed it on that level. But it is definitely the question of Ender's philosophy that is at the heart of the story, and that elevates it above any common story where a kid beats his oppressors and becomes popular or whatever. Plus, many side plots from the book were trimmed in the movie. There was an entire plotline about Ender's siblings on Earth that was an important aspect of the book, but is completely missing from the movie (from what I understand). So just read the book. It's short, and a light read, and the only way that you can really answer your question.
As for Card himself, I don't know, and I don't care. From the book itself, the only one of his that I have read, I wouldn't label him disturbed or from an abused home or anything like that. You can't draw a trajectory from a single point, but that's mathematics, and human beings are much more complicated. I like to read tragedies, and dark and disturbing things, and I was never abused, or from a broken home, or any other such thing, so the material he writes about is no evidence at all.
And why it is a Young Adult story about morality... That's because it is a short and relatively simple and easy to read story, one that adolescents can relate to, in a way, and that makes one question things. What is right, and what is wrong, and where the one bleeds into the other. It isn't morality as shown on tv, that's for sure. Because it asks significant and very real questions, and ones that do not have an answer.
