I've actually been considering writing some fan fic to explain how Viconia became Shadowheart's matron mother.

Like the OP I enjoyed Viconia's redemption story. One of the reasons I prefer the first draft of Shadowheart's character to the much more cuddly final version is that it reproduced what I liked about the Viconia romance - SH's words are at odds with her reactions. (*) Viconia was prickly and she often insulted the other party members. While she occasionally encouraged charname to embrace the Bhaal taint that encouragement was at odds with her reactions at key points in the story.

She didn't try to persuade us to side with Bodhi, unlike Korgan she didn't disagree when we refused rewards, she saved the beggar from the city watch and she was the only evil companion to remain once the party hit high reputation (in pre EE editons), she helped up destroy the drow city and, once you became her confidant, she told stories about her feelings of compassion for her brother and the for a baby.

It was a prickly pear romance - needles on the outside, sweet on the inside. The supportive charname saw the goodness behind the tough facade . . .

BUT Viconia had multiple endings and Larian had to choose one. And remember that Viconia remained a Shar worshiper even after her redemption. (Gaider wanted to make her a Elistraee worshiper but ran out of time)

If Charname refuses to become a god and Viconia is alive Lolth kills her. If she is redeemed she still answers to Shar.

Here begins the outline of my fanfic:

I would have liked to have found Viconia's diary where she reveals that the Shadowheart mission and Vicky's reduction in power was Shar's punishment for falling to corrupt Gorion's ward. After Charname's ascension Viconia is a difficult spot, if V abandons Shar, Lolth will kill her. If she becomes the high priest of Charname then Charname will have to protect her from both Shar and Lolth while protecting himself from Cyric; a battle that Charname is certain to lose (**) A fact that Viconia realizes even if Charname doesn't. So accepting Shar's rebuke and submitting to her divine will was Viconia's final act of love for Charname - she sacrificed herself so Charname could live.

Which is consistent with the story we have thus far: if you ask BG3 Viconia if she is acting on Shar's orders she evades the question; Shar is not happy with Viconia wants SH to replace her.



(*) btw Larian in the circus game I think "kindness to animals" should have been the better answer than "discretion" for white-haired Shadowheart. SH has rejected Shar and is discovering her true nature and should now value compassion above discretion.

(**) 'He' because V is cishet