1. Explained in The AnswerRemco32 wrote:
Didn't the PC die after fighting Nix?
What happened after the PC closed his eyes on the roof with Aigis?
If you killed Rioyi (or how you're supposed to spell it), life just goes one and the world didn't end. How could that happen?
2. Explained in the Answer, ask someone else to tell you for I am lazy.
3.Technically, it does. The ending on that is showing they're oblivious about what's going to happen. So they don't know that the world is in fact going to end, hence the whole partying, etc.