Sooo... Is it only flirting when you're into it?

I love this incel question. It makes you think.

The relationships in poly culture I witnessed are playful enough to have your metamours flirt with each other, or have some banter. Those people share a partner after all - you'd hope they get along.

Some poly couples (primary partners) share a partner. Unicorn hunting isn't well received in the community, but that's not what Halsin and Tav are doing. Tav is having another partner.

Some primary partners expect to be asked before more than a hug happens. Some partners want to know details. Some don't want to know anything about their partner's affairs, relationships, and whatever.
It's up to each couple within a polycule to define the rules. Make sure they work for everyone else in a polycule.

Usually the "don't tell me anything" people are the ones who will become trouble in the future. Hickeys happen. And not being able to share that someone you're seeing is making you happy (more details aren't necessary) is a strain on a relationship. That's like "I don't want you to talk about your sister with me". Uhm, this person is important to me?!

Where do Halsin, Tav, Shadowheart and Astarion falling on that scale?

I'd say Astarion and Shadowheart want to be primary partners. They want a committed relationship with Tav.
Halsin seems to be a relationship anarchist, someone who does not want to label relationships as primary and secondary. Therefore he would describe his relationships as... fluid. Which he does.

There is no contradiction between someone being poly, and someone being a relationship anarchist. There's a difference on the commitment level.

The problem for some folks lies in here. He's non committal. But that doesn't mean he's a sex offender, holy cow! All the companions do is offer, none of them moves anywhere without being told to. Full control in the hands of the player (where it belongs, and yes I wasn't keen on Harleep, btw).

For the type of poly person Halsin is portrayed, his reactions make sense.
For people who are into committed relationships, that sucks.

What I wish would be an option to get into a relationship with someone post Act 2. Some romances could be re-kindled. That would make more sense for people who wanted Halsin, but don't feel like his view on relationships is their cup of tea.

There have been several times in my life where I had to break up with someone because their poly wasn't compatible to my poly. Or their wish for committment doesn't work with mine.
Most have stayed friends. You can love someone, and still aren't relationship material.
There's an inherit sadness when that happens. Still, not even rare to happen.

PS: I know of several polycules who meet for stuff, swinger clubs, kink bars etc. If you didn't find your town square fuckaroo group, you aren't looking hard enough wink /joke