I've tried asking for quests twice (Since it's the only dialogue available) from two separate saves (I think), both times I was directed to Feran Sadri who then gave me the ancient power quest, I haven't actually gone and completed the quest (nor for that matter did I save after receiving it either time). According to the article you linked to in addition to having completed Kindred Judgement getting Destroy the Dawnguard requires you to have completed several side quests for the Vampires, so it seems I wouldn't be able to receive it right now even if everything was working properly since I haven't done any of the side quests yet.
When I input the addtofaction commands they went through (no error message was returned) but nothing changed nor was any indication given that I was added to a faction if their should have been one. To test if it was working I then used the command to take me out of the faction and it made taking anything in the castle be marked as stealing (though oddly no one treated me any different than they were before) so I guess I was inputting the commands right.
No, I don't know nearly enough about the game to guarantee that it isn't one of my vampire mods causing the problem, but the problem is very specific to Kindred Judgement (I reverted to an earlier save before Kindred Judgement and there was no problem again, just as it had been the first time.) and I can't think of any that should be affecting anything around that quest. (The closest is a castle Volkihar overhaul that waits to make it's changes until afterwards but I tried again with it disabled and it still didn't work. And actually I didn't add it to the load order until after Kindred Judgement but before I'd fully noticed the problem anyway. The first symptom I noticed was the two filler vampires seeing me as mortal.) Edit - Oh and there is Marriable Serana, but I don't think it interacts with Kindred Judgement other than indirectly though the Valerica coming back to the castle event that happens after the dawnguard questline is over.
I suppose it's possible that the problem is actually being introduced after Touching the Sky and merely being hidden by the quest specific dialogue of Kindred Judgement, but I have even less that should be affecting that (I think literally the only mod I have even vaguely related to that is an artifact overhaul that includes Auriel's bow.) At some point if no one has a better suggestion I may reload a save from before Touching the Sky and play all the way through to the end again, but Touching the Sky is LONG and I thought I would at least ask around first.