You raise good points. I'd say that Order of the Phoenix was my least favorite book.
Yeah, Rowling wants Harry to be a moody teenager, one who (understandably) cracks a little under the strain. But I think she was heavy-handed, and it looked out-of-character. She obviously meant it be a piece of character development, but I think this largely failed.
I never felt that the "unforgivable curses" were available only to evil people. It's like sin or crime. I wouldn't murder anyone -- but I could. I will say, however, that as character development this may succeed a bit better than Harry's waspish behavior. Harry eventually becomes known for his use of the disarming spell. He makes very good use of this very innocuous spell, in situations where others would likely use more powerful spells. So in the long run, Harry pulls back from his brief use of an unforgivable curse and becomes much more circumspect. Decent character development in my book.
I would also say that one of the messages of the Lord of the Rings is that evil wins if good people hide their head and refuse to take a stand. Harry Potter has this message, as you point out. There's a lesson there for us all, and I don't see any reason to tie it to WWII or Iraq. It's just Good and Evil and it's a universal truism that we need to be vigilant.