I've skimmed over this thread and, while I might be Halsin's #1 Hater (I already disliked him in EA, which I felt was unheard of back then) and I dislike the Drow Twins dialogue in general, I don't get most of the issue being discussed here.
There are certainly a lot of issues with Halsin's writing, most of them caused by the influence of catering to fanservice via the memeing that went on Reddit during EA and the lack of development he got beyond that.

That being said, I don't see how Shadowheart was inconsistent here. She's certainly the most timid of the romances when it comes to her scenes, or used to be before Wyll was rewritten, but when it comes to her dialogue since act 1 she's always been quite flirty (Thinking of her reaction to Tav licking toes or the Abdirak scene). It's certainly not something that bothers me about her characterisation, compared to, say, her weirder epilogue lines that take me out a bit of the game.

I'm not sure if I saw it on this thread or another one, so excuse me if I'm mixing them up, but I also have to clear the misconception that Corcoran only wrote SH for Act 3.
I recently watched this sort of interview stream he conducted with Jennifer English where he talked about how in the earlier stages of EA he was envisioning her as a femme fatale, which is quite far from the final product, and how he originally wrote her as an elf, but that had to be changed because of Astarion's origin bite scene and how she required to be asleep for that, which elves can't be. (There are some pictures of this early full elf Shadowheart around, for those interested! It's just her earlier head mesh with pointier ears.)

Also, I saw someone talk about a scriptwriter and the stool convo as proof Corcoran wasn't her initial writer: from my understanding, a scriptwriter for Larian doesn't actually write the script as we understand it, but organises how the coding of the dialogue branches work. Basically, they handle the more technical aspect of translating what the writer wrote into the game. I might be wrong in my understanding of this job, though. The stool conversation is a very short acknowledgement that you sat on her stool, so I assume it was just a cute idea the scriptwriter suggested.