As of now, Uta is indeed alive. The story hasn't depicted anything otherwise. Maybe there'll be changes in the future, but for now, Uta's part in the manga continues with them being alive.
Sure. '100 nin no eiyuu light novel' is likely a light novel about a hundred heroes. However, without more specific information, it's hard to give a detailed account of its plot. It might involve a story where these heroes come together for a common goal, like saving a kingdom or fighting against a great evil.
I think the themes of friendship and sacrifice could be present. In a group of 100 heroes, they may form deep friendships. And in the face of difficult situations, some might have to sacrifice themselves for the greater good. Also, the theme of self - discovery could be there as each hero might learn more about themselves during their adventures.
I'm not sure specifically as I haven't read this light novel. But generally, it might be about a hero's journey in another world based on the words 'shinwa densetsu no eiyuu no isekaitan'. It could involve the hero facing challenges, making new friends, and having adventures in a fantastical isekai setting.