Except he did though. He was the only person who could stop Shigaraki. Can you imagine the global devestaion he would've unleashed had he not been stopped? It would've been collapse of all modern society in record time.
And there's reason it was phrased the way it was; "This is the story of how I became the GREATEST hero all time" Greatest doesn't mean #1 Ranked. The entire story revolves around what it means to be a hero, and through Deku we're shown that heroes don't always match what's considered the "normal" or "ideal" traits and qualities established by society. By society standards, Deku should never have become a hero at all, he was born quirkless, his early use literally had him breaking bones just to attack, and that's just to start.
But being a hero is more than rankings, and bureaucracy. It's rushing in to without thinking to help someone, it's standing up for you believe in, it's supporting your friends and standing by them when they need it most. And it's about understanding sacrifice, and putting the greater good of those you're trying to help above yourself.
Because he did end up being the only one able to stop Shigaraki, even though he knew what it would cost him and that he would be giving up OFA. The moment he accepted what he had to do until it was finally over, he was the greatest hero in world.
No, because they no longer have the energy to activate or maintain their Devil Fruit ability. So they wouldn't be stuck in Diamond form, they would still drown. As we currently understand and unless Oda show's us otherwise, there is no workaround to the seawater weakness of DF user's.
