The worst thing is, Viconia would've fit perfectly in Shadowheart's storyline. From Viconia's (non-romanced) epilogue, we know this:
Viconia does found a cult of Shar in Waterdeep and she is betrayed by one of her followers. She is, not Shar. Shar, in fact, admonishes Viconia for this, showing she clearly never ordered that. And Viconia does not give a damn. This is the woman who left the cult of Lolth and went to the 'lighter' option that was Shar (compared to Lolth). She already deals with agents sent by Lolth, she's not going to let Shar tell her what to do.
The nature of the betrayal Viconia was the subject of is left entirely up to the player. With Viconia and Shadowheart's backstory being what they are, Shar's plot for a Selûnite child could've been perfectly inserted as the reason for Viconia's falling out with Shar.
Viconia left Lolth because of how children were treated, how she was treated herself. She did not turn to Shar to do the same thing. She would reject Shar's plot the moment the order came, and that would be the perfect trigger for one of her follower to turn of her. Shar's cult being what it is, I don't think taking down the whole lot is overkill to avoid further backstabbing. There aren't many who will pick their priestress over their goddess, especially not with how Sharran's faith emphasizes self-effacing. Can't make a decision for yourself when you don't think for yourself.
Shar's reprimends following what Viconia did further proves this wasn't what the goddess wanted. Complete that by adding she's incensed Viconia rejected her order and slaughtered her willing clerics, and you've got the perfect connection between Viconia and Shadowheart.
In BG 3, we could've met Viconia in Baldur's Gate, rather annoyed because she's been getting an influx of Sharran agents after her, and she's tracking the source to put an end to it. A Shar-aligned Shadowheart could've the mission to purge the cloister that failed to kill her, while a Selûne-aligned Shadowheart would work with her to put and end to the cloister, to give them both some peace and quiet.
Epilogue could've them both sharing tips on 'How to best kill an agent of faith sent after you'. They both have sharp tongues and a similar past, they could've had fantastic banter.
Instead we have this mess.
