Heh, yeah. It's a great game, but still a game.
I don't see that Cain himself, a mystical progenitor no one currently alive or undead has seen in well, ever, is personally pulling your strings over this small scale vampire turf war. And PC generation clearly stays the same the whole game, otherwise it would be natural to assume diablerie as the reason for your success. And the reason the PC can resist the dominate attempt could simply be you passed your last saving throw or LaCroix fumbled his roll.
True, somebody could be feeding you powerful blood to supercharge you, and Andrei could be sensing that. Not sure if that would have effect on the dominate as it doesn't actually lower your gen. And Andrei could be just saying in his own unique way that you are stronger now. Or he could be sensing that bloodbag of elder blood you found (among other places) in his fridge and chugged down without a thought of consequences.
All this speculation is fun, but in the end it's probably just a standard of a crpg - zero to hero, you grow powerful over the game until you can take everyone on and win. Being an 8th generation would certainly help a little, even though it also raises a question over why was your sire that low gen. Maybe he did engage in a round or two of drain-the-elder, possibly explaining why no one really objected over his execution? Who knows.