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.
Edit: And I know it's wild coming from me but maybe we could get back to Wyll now.
