Thank you for the information! I did not know you could get the special endings only when you are in the same house as your mentor, I must have gotten that right by accident and so I didn't notice. (Might explain why the red choices didn't work for another user.) Truth to be told, that's a little bit lame, especially since it means that the final "true" ending is exclusive to Golgotha vampires, and I already have the feeling that the game is tilted in favor of Markus/Golgotha. Don't get me wrong, they are my favorite house and character too, but it still feels... slightly unbalanced?
Edit: Finally managed to start my own rebellion on Randal's route, yet his second ending still eludes me. Might be a bug, or maybe there are some other conditions? Second edit: I finally got it by siding with Andrei, but then letting him get murdered by Lazarus. (Must have agreed to Lazarus' deal beforehand.)