I have managed to unlock 12 of the 13 endings linked to the mentors, and while this is not a guide, maybe it might be of help to some people. Spoilers ahead: Basically speaking, every mentor has an ending linked to one of the factions. Those are usually pretty depressing, ending with the mentor broken, vanished or dead, except for Markus, who always does his vanishing act. Once who have reached some of the endings (the game will tell you exactly which endings you need), you will unlock the final ending for each mentor. Once you've done that, some of the choices will be marked red, pick those if you want to reach the special ending for each character. The very last ending you can unlock is Markus' final ending, which is probably the the closest to a "true" ending and the only one that counts as bittersweet instead of just bitter. It also makes clear that yes, our choices so far mattered, though I guess it might be too little, too late?
So we have nine mentor/faction combinations and their three special endings. Together with the two common endings (I got these without paying attention, but I think you get them when you mess up and get killed) that accounts für 14 of 15 possible endings. The only ending that eludes me is Randal's second ending, maybe that's for starting your own rebellion on his route? (I think for the other two it doesn't matter whether you go Mavvar rebellion or make your own rebellion.) Not quite sure what I did differently to get the option to start my own thing, so I can't check easily. If anybody managed to get this ending, feel free to enlighten me.
PS: Randal's special ending was SCARY. How scary, you ask? Scary enough to frighten Andrei, of all people.