[Hiroi Sekai]

Naw, I'm assuming they'll go with another, since they did leave enough open to do so. You'll know you got all the endings when:

Spoiler for EDs:

You travel back to before the outbreak, where you're in a fortified bunker with Akane.

Also:

Spoiler for Blick Winkel:

Blick Winkel was the reader of Ever 17, another popular VN that runs decently similar to the 999 series. In that game, Blick Winkel existed in the 4th dimension and could transfer information between storylines to help them progress, kind of like how you time jump in this game.