The credits music for the canon ending of Drakengard is an expansion of the "end of mission" theme from the game. I can't believe I never noticed that. - Zephid
While we're talking about Drakengard, each time you beat the game, you unlock new missions that can lead to new endings. These are interspersed throughout the old content. Each ending is worse than the last: a game designed around completionism is intentionally punishing the player for being completionist! - parrellel
Well, that seems to be a definitive trait of Yoko Taro's games. NieR did something similar. Only time will tell what kinda trick Drakengard 3 will pull.
Why did Inuart's attempted resurrection of Furiae go poorly? The Black Dragon wasn't with him, so he couldn't direct Inuart (into generating Nowe, but Inuart didn't know that). Why wasn't the Dragon there? The player kicked his ass (within a short time limit) in order to unlock this route.
Partially jossed: according to supplementary material, the Seeds of Resurrection/Destruction are designed to transform what is put inside them into a monster/Watcher/Angel that will destroy humanity. The Bone Casket is a giant Seed that the Dragons can control. It's still Inuart's fault that he used the wrong Seed, though.
In Drakengard 2, it has been noted that destroying the Bone Casket, which contains much of the Gods' power, will be a hit for them. This is valid for any pawn of the Gods. Think about it:
1st game, endings A and C: defeating/eating Manah while the apocalypse is going on seems to temporarily stop it.
Endings D and E: Manah is killed before the end of the world, when she hasn't got that much of the Gods' power in her. The Gods recover and immediately deploy plan B - the Grotesqueries.
And in ending D, the Grotesqueries get frozen in time, along with whatever power the Gods gave them. That's why the rest of the world is "saved".
2nd game, ending B: the Casket is destroyed alongside Manah. The Gods deploy the Grotesqueries again, but it's implied that there has been enough time for the humans and the dragons to build an army and fight back.
Ending C: Manah absorbs the Casket and the Gods' power, refusing to use it. The Gods are effectively trapped within Manah.