Okay, but for me this is insufficient (a weakness of the game's writing), because a more reasonable path (for me) would be turning SH away from Shar but then making her truly pay for any and all evil she's done (i.e. atonement without any reward for the atonement).
I can't say for sure if the game will do the things you'd like it to do. What I can tell you is that redemption in the religious mindset you think the game assumes (and I'm not so sure that's the case but we'll see) usually is a reward onto itself AND more often than not it implies personal sacrifice not some form of material reward. Take the Lord of the Rings (a book that certainly shares the mindset you are talking about) as an example. At a certain point one character realizes he did wrong and redeems himself by allowing his companions to escape while he stays behind and protects their retreat. He did redeem himself in the end but he paid the price of redemption by dying.
That may very well be the case with redeeming Shadowheart (although I'm not sure modern players would like that outcome...) also keep in mind evil gods in D&D do not like when their faithful leave them and often try to make an example out of them.
Fun fact:
As Killer Rabbit says in BG2:Throne of Bhaal you get the chance to redeem Viconia, an evil NPC companion, drow cleric of Shar changing her allignment from evil to neutral. If you do so and complete the game, a few years later Viconia is killed by agents of Lolth, the evil drow goddess of spiders she was a former priest of before leving her worship for Shar's. Given the similarities between Shadowheart and Viconia (ok, Viconia is more evil and way hotter...) I don't think it would be a stretch for SH to get killed if she changes her allignment
