the reason for the multiple endings is to conform with the players opinion on how the story should end
This. The idea is that Horatio's motivations and worldview, particularly by the end of the game, are fairly open to interpretation, so I tried to give a range of endings that would let the player define Horatio (and thus, to some extent, the overall theme and values of the game).
That said, we're doing a small spin-off thing to Primordia and while it's not a sequel, per se, it presupposes that the player did some ending other than the "mission accomplished" / Thanatos-the-world ending. But players shouldn't feel that the spin off project is sufficiently canonical to trump their way of resolving Primordia -- i.e., it doesn't mean that the Thanatos ending is illegitimate, only that the next story I wanted to tell depended on that not being the ending.
