More design problems is what I chalk it up to. Karlach was added around as late as all of Halsin's act 3 mess. And I think this is the point we most disagree on. I don't think the inclusions themselves are inconsistencies, what I think is that it all needed way more time in the oven since the team all agreed to go in this direction with it but weren't allowed the time to let it cook properly. Halsin is a little different where the writer just treated him especially poorly on top of being the extremely ill-considered focus of all the poly; and to that end Halsin is the only real "hidden fetish" the topic title mentions, I don't read that type of thing from any interactions that don't involve him.
It was likely encouraged exactly to prepare them for more specialized training in seduction. The ring thing is about what a fully trained Justiciar did and in the time frame of the game, Justiciars aren't exactly around in force. With how selective becoming a Justiciar is supposed to be I wouldn't expect a bunch of initiates are going to get that specialized training unless it was specifically expected in the selection process which it doesn't seem to be (at least for the specific cult we interact with). And as I mentioned, it's not the only time she mentions familiarity with a quick romp. She just isn't telling us the stories and I don't think that's particularly strange. Lots of people, even those who are quite flirtatious, don't enjoy talking about their past sex lives in detail even if they mention it in passing. There is an upper limit into how much exposition a videogame character should be expected to have, and I mean that's subjective so I don't fault anyone for expecting more, but I don't think it's a fault of the writing that we aren't getting the stories because I don't think they're needed to justify anything. I think the implication alone is enough with all the other details that can be connected about the cult.