The more I think about it....

[Zaffre]

I would say that Agrippa's ending is generally accepted as the true ending to the game, because a) Alexander's ending makes no sense (why would Daniel choose to fail after going through the entire castle?) and b) Daniel's ending makes him a douchebag to Agrippa.

I think Daniel's ending makes some sense. If I woke up knowing nothing in a castle and was expected to save a stranger I never even got to know, I would probably

Spoiler below!

knock over the pylons in the Orb Chamber

unless saving Agrippa was vital to my survival. However, I never found enough notes to give evidence that I should've saved him. If there is evidence, then I'm mistaken.