Going into whether the "true self" of someone is something that exists as a fixed given in a vacuum, instead of something that develops and is revealed through interaction, is probably a bit too philosophical for this thread. But if we approach the topic more from a "what a character truly wants" angle, your decision to attach this to what they say about themselves at a fairly random point in time and despite them being shown as unreliable narrators from the go, is way to arbitrary for my liking.
In Shadowheart's case, she says she's Lady Shar's number one fan, but you are shown early on how brittle Shar's sway over her is and how much effort Viconia and Shar have to put in to keep her from discovering things about herself which conflict with her Sharran identity. So it's no wonder that it takes very little to make her question her faith.
As for what you would like companion's decision making to be, that's pretty much Gale.
I agree:
Shadowheart was never really a Sharran - she cares about the wellbeing of children, animals and innocents too much. She feels bad, if the group attacks the grove.
Lae'zel is outright benevolent for a githyanki, she is at least open to learn about Faerun and befriends some of the companions, especially Gale, Wyll and Karlach, very quickly. She even shows admiration for some of them early on (Karlach, when recruited, Wyll, when he stands up toMizora and gets changed for example). That is not your typical gith behavior.
Astarion I will leave out, because to not bring another discusiion in here and because of forum roles, but feel free to pm me about that, if you want to hear my take.
Wyll is generally open to councel and takes critic very well. He also offers advice himself, so I think, he would at least listen to your arguments. Which I do like in the game, still he should be the one who tell MIzora, if he is in or out.
Gale is like Wyll someone, who listens to good arguments and since he relies on you helping him with the Crown and since he is normally teh first one to get approval up, I think, it fits that he would listen to your council.
With Karlach it's a bit different: She will only listen to you, if you go with her to Avernus or she will listen to Wyll and join him in Avernus, but otherwise, she will choose to die or try out living as a mindflayer.
I would say all of them have the potential to become their 'true self' according to what is already in the character or become something they might have think they wanted for various reasons, but doesn't necessarily make their life better. I bet that Sharran Shadowheart is not happy in the long run, since that is totally against her character. Same with Duke Wyll Ravenguard - that is not, who he is and wants to be, but still, both probably can live with that decisions. Lae'zel on the other hand is doomed, Gale might be too, depending on his further decision.
It is an interesting topic, that was brought up, but I do think, it should be its own topic in the story section.
