I have played the early access. I say this and probably if i am right someone will remember this words.
I think Shadowheart worship of Shar is not genuine at all. I think she is indeed a cleric but nof ot Shar. And i think she is costantly brainwashed. There are many instances where she show genuinely a good side and she show to care. However i do believe she is a quite a lot conflicted character and part of her true self can surface out if not all if she manage to recover the memory.
To me she is just Brainwashed with no memories. And her reactions with the temple of Selune... Well that speaks a lot to me.
((would also explain why her approval is inconsistent at time. ))
