In my game Gale offered a magic lesson during one of the long rests before the party (when I caught him looking at the image of Mystra). So I could totally imagine Astarion standing in the corner, watching and being jealous, especially since it happened after the first bite.

Astarion offered to spend the night together shortly afterwards.

During the party Gale was acting like there was something going on between us (I chose the option to hold hands in the Weave) despite me selecting only neutral and friendly options during that conversation. He talked about his orb being too unstable and that he shouldn't get excited, then said something about having to wait for better times. I had an option to think about him during the party night but it led nowhere when I tested it. Then he kept ignoring Astarion romance until the starry night date and I could only reciprocate his feelings, kiss him or say I didn't love him. There was no way to say you were into Astarion, Gale behaves like he doesn't exist. He only has something to say about him in the morning if you followed him into the illusion. It doesn't matter if you slept with him or not, he acts as if you had sex, which is the same inconsistency during Halsin's proposal.
Oh yes, I always imagined Astarion pulling out his knife too! It was strange that he initially had no reaction to Haarlep, especially given his past. I still think he's too calm, but I doubt anything will be changed in there.
Yes, Gale was looking at the image of Mystra, but in my playthrough this scene happened much later - after the party, and at the party Gale offered a magic lesson just like that, without talking about Mystra. The party is where relationships begin, and even though I was already in a relationship with Astarion at that point, the abrupt breaking off of the dialog, with no way to refuse Gale, made me paranoid. I later read that Gale could ruin the romance with Astarion at the party and it was advised not to approach Gale at all, but as you can see from my experience, that won't happen if you just approach Astarion again (the dialog with him will restart). By the way, I made an approach to Astarion three more times for prevention, and each time the dialog starts again, so you can see different variants of dialog with him without having to reload. To imagine something adequate, of course, with this sort of thing is not easy, it's something like:
Gale: I'll come to you tonight and teach you magic.
Tav: Nah...
Gale (covering his ears): Blah, blah, blah, I can't hear you... I'll come to you at night like I said.
Tav (after two seconds): Astarion, will you come to me at night as you promised?
Tav (after ten minutes): Astarion, don't forget, you promised to come to me at night. Write it down on a piece of paper so you don't forget.
Tav (after thirty minutes): Astarion, do you remember that you promised to come to me at night? Here, hold the piece of paper I wrote for you so you wouldn't forget. Don't lose it, and don't drink too much, or you'll forget. I'm watching you.
Tav (at the end of the party): Astarion, aren't you forgetting something? Where's the paper? It's okay, I'll just stand here while you finish your drink. (looking back at Gale)
This is pretty funny. Maybe if the scene with Gale had happened before the party, I wouldn't have thought anything of it and would have agreed to the magic lesson because it sounds interesting on its own and doesn't even seem like flirting. I guess it depends on Gale's approval level, or I just had too little rest in the first act. I decided to turn him down in the second playthrough as well, and I see I did the right thing since he acts like you had sex. This is an extremely odd inconsistency in romance, the player's decision is ignored, the companion acts like the other doesn't exist. I managed to avoid this unrealistic behavior with Gale, except for that funny incident at the party when Gale “doesn't take no for an answer”. In your case, Gale comes off as a very brash, narcissistic, and self-righteous suitor who seems to ignore Astarion's existence or considers him a “temporary distraction” and himself the center of the world and the God of Magic (in advance).
Unlikely to change, it seems like the game is more important that all players have the opportunity to accept Haarlep's offer.
It's bad. I also wish for an option to tell Halsin to shove it after you're done gossiping with Astarion. I would have liked to make fun of Halsin with my LI too.
Oh yeah, that would be fun! In general, Halsin doesn't represent polyamorous relationships very well in my opinion. A polite person should first ask how the couple they want to join feels about polyamorous relationships. And only make such an offer to them if they respond positively. Behavior like Halsin's, where he simply ignores the fact that Tav has a couple, and except to “graciously” allow: “Well, you ask him for his consent, I can hook him up too, it's a bear thing to do...” causes a negative attitude towards Halsin's character and a feeling that they are literally trying to impose polyamory on you. There were plenty of opportunities for polyamory in DAO and Pathfinder too, but it was seen purely in a positive way, as good opportunities for other players, without any negativity towards it. Halsin himself is not a bad character, I ran with him for interest in Act 3, and he has normal lines (as a companion, he's nicer than Wyll and even the aforementioned Gale). All it would take would be to make Halsin's behavior less intrusive and defiant, so as not to form a preconceived notion of the character, and he wouldn't be seen as a “rattle for Orin”. I've changed my attitude towards Halsin, but his romantic scenario clearly leaves a lot to be desired.
I could only trigger Wyll's dance if I kissed Gale. Otherwise he seems to think you're loyal to Astarion and doesn't bother. All in all, Gale is the only one who's clueless (or determined) enough to push the romance.
I haven't kissed Gale once, but Wyll doesn't mind. He asked me to dance right after we freed Mizora and I managed to get her to promise to get Wyll out of his contract. He makes those unhappy puppy dog eyes after you say no, why? It's just ridiculous. Astarion's confession happened exactly the next night.
Not counting the newly debugged banters, only the women notice the romance, the men act like nothing's going on until you get on with them and they ask you to choose one of them. It's strange.
Absolutely right. Men act like males who don't notice the competition. Women are much more calm. Lae'zel is an exception, but she always manages to propose before Astarion, I'm not in a relationship at this point, so it's normal. At the party she talks about what I'm losing, but that's in the past tense. Shadowheart sees our relationship with Astarion, and Karlach doesn't say anything about our relationship, but also doesn't try to get hooked up with Tav in any way. At the party, she says she'd like a “hot guy.” Although I know she has a scene where she tries to seduce Tav after dealing with her inability to touch, but I didn't have that scene.
He admits himself during the epilogue party, if you're not in the relationship and are rude to him, that he liked the fact you were challenging him, so he definitely doesn't mind if you make him work for your affection a little more and tease him. ^^ In the post-Araj confession he still makes fun of you for being an easy conquest, despite all the obstacles you'd put in front of him.
Interesting. So “easy conquest” doesn't change.

For such a seducer, it seems any conquest is “easy”. I love that line. It's especially fun to hear it when Tav herself has hunted Astarion before, and “ lured” this seemingly untouchable biter. Teasing during a night under the stars, hey, take the book, Astarion, look how much HP I took down off Gandrel, Astarion... “What do you mean, ‘shit’, Astarion? You haven't even tasted it and you're already criticizing it...” (just kidding, of course, but I wouldn't mind such a fun line for a bite scene). And finding myself an “easy conquest” afterward.

A little while later Astarion woke up as an Ascended and a married man without noticing the catch, absolutely voluntarily, he wanted it himself and sincerely considers himself to be in charge of the family. I can roleplay a pretty sweet “gotcha who bites” romance story with that “easy conquest” of his. Astarion is a very reactive and versatile character, he is great at building relationships with different characters, he loves a challenge and a game, but he also appreciates just a good treatment, he can fall in love with both the bright and bold and the inexperienced in love science Tav. In the case of choosing only the most positive of possible lines, some of his incredulity is evident at first, like the line “So cute...” said in a decidedly ironic and incredulous way in response to the promise that we will definitely know the meaning of his scars. Maybe teasing is something even more familiar to him, he's pretty sharp with his tongue himself, and in this case Astarion is kind of unsure how to respond, or thinks this behavior is possible manipulation and doesn't believe in Tav's sincerity until Tav can prove it with deeds.