Shadowheart really could do with being a lot more subtle, as is if I was DMing BGIII I wouldn't event require a religion check to figure out what she's a cleric of when the party meets her if the player in question was also a cleric* and would give a fairly low one for everyone else. Shar is probably *the* most well know evil god(dess) in the setting. Any cleric walking around with that regalia, with that name, and talking the way she does (particularly in regards to Selune)....you are way past foreshadowing (ha) at that point.
There was a suggestion that she change her outfit into her Sharran regalia only after being uncovered-I think that's a good start. There was IIRC a datamined hairstyle for her that replaced her hair cuff with a leather cord. Have her start with that and some more generic armor or the armor of a priest of another faith that she could be in disguise as.
*Honestly Clerics should autopass or have reduced DCs or advantage on a lot of Religion checks. Recognizing basic things like holy symbols or the trappings of major faiths should be an entry-level requirement for a lot of organized religion in the Realms.
