But it doesn't change my visceral reaction that the movie is all sad and stuff when it's not boring.
Oh, of course -- I'm definitely not trying to convince you to like it. The movies work for me, but as noted my view of reality is on the bleak side. And if we all felt the same way, it wouldn't be a very interesting conversation.
So here's how you're wrong... (Kidding!)
Batman's not just taking the blame for Dent's death, he's taking the blame for the people Dent killed. And that actually helps him with a couple of problems: amateurs are imitating him, and bad guys have figured out that he’s got rules. If everyone thinks he’s a murderer, it stops what remains of the hero-worship and scares the criminals. And I don’t know that trashing Dent’s reputation would save Batman's -- his reputation takes some hits once the Joker says he'll keep killing until Batman unmasks.
I don't think there actually is a lot of time spent on the cell phone hack. But I get that it feels long, because it is pretty silly. Basically there's one scene of Fox harrumphing about how this is awful , and Batman’s all, “you’re the only one who can work it, so FACE” and then Fox says, “I’ll do this one time, but then I quit,” and then at the end the thing goes kaboom and Fox is like, “Oh, what a delightful prank.”
On Dent’s importance -- It’s stressed repeatedly that if Dent’s seen as yet another corrupt official, all of his prosecutions will be tossed out. And there’s precedent for Gotham loving martyrs, since the death of the Waynes apparently horrified people enough to halt the League of Shadow’s first attempt to smash Gotham. (Through... methods?)
I am amused that in BB the LoS sorta comes off like Wile E. Coyote. “We had this complicated long-term plan to accomplish our goal. It seemed to be working perfectly, but a one-in-a-million event caused it to go wrong. Instead of trying that again, we’ve moved on to a different, even goofier scheme!"