Shadowheart is actually way nicer, and I seriously don't get complaints about her being evil. She doesn't do evil things either, she approves of good deeds, and in her romance scene she is incredibly vulnerable.
She's a cleric of Shar. That makes her evil. Period.
The way Astarion is technically evil alignment wise, yes. I think people mean, though: "This is not the real Shadowheart. My proof is that she approves of too many good actions. Her true self is bleeding through the brainwashing and memory implant."
You're arguing for the state of her, some people are arguing for "the real Shadowheart". That's two different things. You're right, but their theory also has support in data mining and what not.
