Come on. If you're going to make such a big deal over the "parabellum" idiom, at least use a gun chambered for a Parabellum cartridge, rather than a Webley.
So it's confirmed there's multiple Oukas and Chihayas, some at the same time, some in the past/future, and also inception, and time traveling (possibly)
So the comet that destroys the world is the bullet that Ouka fires into her own head. Still a bit confused about everything else but that's to be expected