The eternal battle between "Shadowheat is evil because she is a Sharran" or "she isn't evil because she doesn't disapprove of the outcomes of a goody-two-shoes PT"
By being a Sharran, she has committed crimes in the name of her goddess. There is no "we haven't seen it, so it didn't happen" here. You aren't a member of Shar clergy, like Shadowheart, without committing atrocities. Refusing to do them would get you killed or worst.
Did it happen a few times that her heart wasn't into it? Maybe, it's not like evil characters are all robots. In the case of Shadowheart, she feels bad about the Tielfings, yet she doesn't seems to care about the Druids. Wouldn't a good person feels bad about both?
Was she evil before joining the clergy? Probably not. The people who join Shar willingly usually suffered a great loss (major depression just want to see the world burn type). Otherwise they were forced into it via destruction of their previous self (using torture, gaslighting, abusive manipulation, etc). I did say that what she needed was therapy and not a redemption arc. It wasn't a joke.
I said it before about Baldur's Gate 1-2 and I say it about Baldur's Gate 3 as well - if it's not in the game I don't care. Not because I want to spite anyone or because I don't think FR lore is important. I just don't think a game should force me to research outside sources to understand its story or characters. It was kind of a problem in Baldur's Gate 1-2, but not in any other good Rpg I ever played (Dos 2 maybe, but Larian is known to have its lore all over the place). If this bit about Shar initiation ceremonies is important enough that it affects what kind of character Shadowheart is, it should be in the game. It's not a criticism against you, but against Larian. Either they decided to ignore it, or they missed it, or, as you say, she is a really evil lady with a strange fondness for dogs and tiefflings. the only reason to know for sure is to wait for the final release, but if you are right, her character is not only bad but also kinda badly written.