Regarding alignment, it doesn't necessarily have to be something that is made known to everyone. It doesn't have to be a tag, or a visible tag, but it is something that the DM (or the game) can use to identify your actions and make reactions in response, and it's something that the player can use to guide their own roleplay. Alignment is a fluid and mutable guideline that informs your personal moral outlook. I personally see it as something to look to when I consider the choices I'm going to make. Without it, I'd be less inclined to take the character's personality into consideration when making choices, and simply use my own best judgement, which may not be how the character would act. Creating an understanding of that character's general outlook between the player and the DM has a lot of benefits.
Allowing alignment to change based on a player's actions is great. It should be allowed to change. For a good example of how this can work, check out Pathfinder: Kingmaker. But just because it can change, that doesn't mean that it's not valuable to have in the game. Not only does it have direct mechanical effects, but more importantly, it forces the player to think more critically about their character's actions.
I think it just leads to meta-gaming. "I want to stay X alignment so I will do 2 Naughty things and 4 goody things". People will almost always act in their own best interest unless they have some sort of mental health issue such as impulsiveness, or a sexual attraction to violence and a need for control.
From a logical standpoint Halsin is the better bet in regards to finding a cure for your condition. From a logical standpoint working with humans and tieflings is safer and more profitable than working with gobbos and drow. Logically it makes sense to not to use the parasite as it co-opts your brain and thinking. These considerations transcend good and evil.
Astarion doesn't want to lose the parasite - not because he is evil, but because its his only chance at freedom in 200 years.
Then there is the what is evil mean question. Is it selfishness? Because that covers all humans. All human actions are inherently selfish.
However if I show up and tell you I am a Cleric of Bane and my God offers you "Bane's Freedom", then you know I am dangerous, that I am motivated by religious fervor and my alignment is irrelevant. Run!
People will metagame and min/max everything if that is how they choose to play. There are players who see the game not as a tool for storytelling, but as a power fantasy where the only thing that matters is how much damage they can deal in a round. If you're going to try and manipulate the system, then every part of the game exists for you to manipulate, so you can't argue that alignment encourages metagaming because those players would do it with every mechanic. You can accommodate those people, and even support them, but that doesn't mean the system is built entirely for them, and it doesn't mean that alignment isn't worth implementing correctly in BG3. Alignment has always been a part of the game and it's a lot of things to a lot of people, which we are illustrating by having this discussion. I think it's still very important.
As for your examples, I don't see how they show any problems with alignment as a game mechanic. Yes, these characters have multifaceted beliefs and motivations, but when I read these lines, I can still see Lawful, Chaotic, Neutral, Good and Evil. There may be nine defined alignments, but it's still a spectrum, and it's still subject to change. Those people who are mentally unstable and act unpredictably/without regard for law and order will fall under the Chaotic end of the Lawful/Chaotic spectrum. If they consistently do bad things that hurt other people, they will fall under the Evil end of the Good/Evil spectrum. If they make efforts to change their ways, then their alignment will shift, and that's totally okay. The alignment system simply informs the game, the DM and the player about who the character is, and if the entity who runs the game chooses to consider alignment, it can have effects on the game that the player or may not be able to predict, which creates opportunities for interesting situations that I want to have in the game. To say nothing of the actual codified mechanics that exist which relate directly to alignment.
