I disagree with most of them being manipulative.
I haven't romanced Shadowheart, but I did reveal her secret and had her in my party for several playthroughs and I do not find her manipulative. Very secretive and likely to bacstab you? Propably, but honestly - all of the characters are. And I really like it because it invites some interesting dynamics and so much potential for gazillion playthroughs.
Lae'Zel is just doing what she thinks is best, I feel like especially after 1st tadpole power she is friendlier, although approval might have triggered that as well
Anyways, she warmed up to me, both of them did. And I don't feel the looming gloom as much as with Astarion.
I've read that theory about Gale! I think it's very likely there's more to him than meets the eye. I must admit I am not very interested in having him around once he ask to eat relics. What if I am feeding some evil god?!
Alright, let me unpack this: everyone is entitled to their own opinion. Now let me explain why I think they all are manipulative, aside from Wyll.
Lae'zel - when she's stuck in that cage, she literally makes the PC believe that she has a cure to the tadpole problem. Well, I wasn't shocked when it came out she was bluffing. Second strike is after meeting Zorru, you have to pass an insight check 'for signs of deceit' if you pass it, she's legit lying and having doubts about the creche, you can use the tadpole on her to peer even further beyond her barriers. Which again, I did out of curiosity, but reloaded immediately after. She knows deep inside that the purification is most likely bogus and will end up in entire groups' death, but she's githyanki, she's been legit brainwashed into believing things like: purification will save you, she's still trying to manipulate you into getting her way, regardless how you look at it. It doesn't make her any worse of a character.
Shadowheart - yes, sure, there are multiple points in the game where you can confront her about her being a Sharran, but her admitting that (under pressure no less) doesn't mean she's not manipulative. Her mere hatred of Lae'zel and constant remarks 'you shouldn't take her with us', 'you shouldn't have freed her' etc could've been written in 'manipulation manual for dummies 101'. Manipulative people can be nice btw. Again, it's not about characters warming up to you, it's about the way they act and react to different situations. And no, I haven't romanced Shadow either, not planning to. Basically what @azarhal wrote:
Shadowheart is a Sharran, she should be manipulative, it's one of their main traits.
Shadowheart might be the nicest, most kind Sharran (what is a contradiction in itself) on the face of Faerun, but it doesn't make her any less manipulative. For all we know she could be using us to smuggle that 'mysterious artifact' to Baldur's Gate undetected, back to her Sharran priests and who knows what kind of catastrophe it could cause?
Gale - I think his topic is exhausted in that reddit link I posted before. :P
Astarion - also manipulative as heck, he does his best to gain PCs trust to basically manipulate the main char into becoming his little mobile blood supply, shall I say more? Still, he's my favorite of 'em all, mostly because of these little moments when he breaks out of character, when he's absolutely terrified of Cazador's reach, so much so he temporarily drops the mental walls to put them back up again.
Wyll - he's a special case. He's clearly conflicted about the deal he made with Mizora, cause he knows deep inside that his hero 'shitck' is a lie, he's been lying to himself worst of all. He knows if it wasn't for his pact, he'd be dead, forgotten and he damn well knows he would most likely accomplish nothing without his warlock powers. Of the entire cast, he does lie and dodge the truth the most, but he's not manipulative, but there is potential... to let him succumb to corruption or fight it. It's quite poetic really.
I legit think Astarion is probably the most honest LI in right now
To be honest I thought this up until I went to the hunter without Astarion. I don't entirely believe the reason why the vampire hunter is after him specifically (discussed it somewhere above in spoilers). My main reason is that the hunter doesn't know how he looks, which seems like a major oversight someone as scheming as Cazador sounds.
If he is truthful about that, than I believe that Astarion is indeed the most honest LI. If he is lying about it, than I am really curious what the reason could be.
I think you are fixated on the hunter a bit too much.
Like I wrote earlier/in the other topic - the reason for the vampire hunter being there is simple. Cazador wants Astarion back. Why? Well, think about IRL history - what happened to runaway slaves/indentured servants? Do you really think their 'owners' would just let them go and not hunt them down to make an example out of them?
Also, if you romance Astarion with a tiefling, after the night spent together you can comment on his big scar 'poem' being written in infernal. Now why would Cazador write anything in infernal? Most likely in hopes of owning Astarion's soul? A soul transfer spell a'la what Irenicus had done to Gorion's ward in BG2 that made the slayer emerge? Be my guest at the guessing game behind the meaning of Astarion's infernal big bad scar. We simply don't know yet.
And of course the hunter knows only a vague description of Astarion, I have a suspicion that the hunter was setup to fail by Cazador. The first one was just a warning. I'm pretty sure once we meet more hunters going after Astarion - they'll actually mean business, be better prepared.
There is so many questions, as to how the story will develop and it's also core to understanding the possibilities of him backstabbing you... Because of how power hungry and broken/damaged Astarion is, he shouldn't be completely trusted.
If the tadpole stays in our head the entire time in BG3, than I trust him, mostly. This would solve the issue of Astarion leaving the party in case you help him with his quest - whatever that is. Without a tadpole and being true vampire. I don't see why he would stick with you, as he would likely burn in the sunlight.
He is likely to manipulate you to help him kill Cazador and once he achieved both goals, he is free. He mentions maybe having his own coven in one of the dialogues I believe. He also talks about how it never happens, that a vampire shares in his power with a vampire spawn. I believe he is honest with both of these as his goals.
If we can manage to control the tadpole powers and unless there is some sudden power shift and he feels like he is on the losing side I think we are all good.
Astarion doesn't say he wants his own coven, he merely explains why a vampire wouldn't give up a slave to become his competitor, he even says he doesn't care about becoming a true vampire, all he cares about is Cazador painfully dying.. And when he says that it's one of these moments when his voice actor's delivery is amazing, his voice even slightly cracks. What makes me believe that voice delivery was deliberate and that he actually meant that. On top of that, in previous topic I mentioned that there are magical ways for vampires to be able to walk in sunlight. Again, look up Hexxat from BG2EE. She has a magical cloak, if you don't wear it during daylight, she slowly burns to death (similar to Astarion taking acid damage in running water). So who's to say that the PC and Astarion won't find a way to make him walk in daylight after the tadpole is dealt with?
And I highly doubt, if treated kindly, that Astarion will flip a traitor switch and decide to betray us for funzies. At least if we follow the logic of - for every action there is a reaction. But if you provoke a hornets nest, you're likely to get stung.
But again, it's all speculation anyway. Things might get rewritten or slightly/completely changed.