Shadowheart? I think she'd be willing to betray us as well, though less likely than Astarion. Astarion I feel would betray us for a broad scope of reasons while SH is more focused on her mission so unless her mission calls for it, we're probably fine.
i think you overestimate shadowheart's faith to shar.
player character -- "do you worry abut helping tiefling that violate shar's teaching?" shadowheart -- "yes, i do worry about it perhaps, but let us put aside it now, i can repent it later." shadowheart -- "let's focus on the celebration."
acturally, i think shadowheart isn't like a shar's believer, because she is not like a "evil neutral" one from the plot of early access stage, shadowheart is like a "neutral chaos" mix a bit evil.
