I'm not saying answer every question, but you DO at some point have to answer the big over-arching question. It could be success, or failure, or after 5-6 seasons being caught in an endless loop, but you want an answer. And in this case, you want something answered. The only answer we got was what caused the end in the first place.
As for the person who did/didn't die, well, we assumed they died. Cardinal rule of TV/movies, no official dead body, no death. Room blows up with someone in it and you don't see person blown up, odds are they survived, regardless of what was implied.
Cha-cha and Hazel can't simply kill off anyone outside their targets because there's no telling what unintended consequences that brings. 3 days before the end of the world, probably nothing. But at the same time, they kill a couple of the kids, then what's his name trying to turn her against them doesn't necessarily happen, or work, etc. You can't tip the wrong dominoes and generate the same effect. On the flip side, the kids aren't really killers or don't want to be.