There's a bit of ace intimacy in Shadowheart's party scene, but I'm worried that it'll eventually become "oh you just have to work harder to get her to put out" type deal that most games would do with that. But here's hoping otherwise.
It would be also nice if they had leveraged the ability to do conversations with characters other than your personal character to expand the varieties of romance types. Namely: polyamorous romances that aren't wholly revolving around a single character. With games where all convo happens from one character perspective that's a bit understandable if disappointing. But in this, I can have my PC talk to Alfira and then Shadowheart... and then have Shadowheart go and talk to Alfira... and
(hmm, I'll have to see if you can provoke one of those conversations that happen in multiplayer by having a companion in your control talk to your PC who is currently botted.)
Anyway... if the tech were in then I could build up Shadowheart's approval with Alfira, Alfira's approval with Shadowheart, and my character's approval with both and then have an actual healthy poly relationship where all three are together rather than past poly where it feels very harem-ish or "I like you so much that I don't mind 'sharing'."
Though, again, that's probably effort they're not willing to spend...
I believe Shadowheart scene is the way it is because
As for your idea of poly lemme say this. It's not worth the effort, it will never be good, I think Pathfinder Kingmaker was the closest I ever played that had this option explored, but in both options, the sisters and Octavia and her partner, was disappointing in the end. I'd prefer if they don't try to mess with this, I mean, my guess is that Lae'zel will have an open relationship (willing or not for the player), she doesn't see the player as her partner but just a toy that belongs to her, and she may want to have more than one.