I've really enjoyed this game! It is very grim and like you say it would have been nice to get some happier endings with Randal and Heath. In my own headcanon I at least think in the endings where they stay with you, as vampires you have eternity to try and make things better, it's only been a few months in game.
In answer to your questions:
1. I'm not certain, but it only happens if you have Randal as a mentor and he's close to you, which makes me think it's him.
2. I definitely thought Golgotha because of all those drawings he does, both the weird figures and nonsense words. I didn't pick up on the lies thing but that makes me think Gol too.
3. I don't know! That's what I googled to end up here :-) It does mention at one point he makes them bleed by digging his nails into his palms, maybe be does that in his sleep?