It occurred to me just now that your party's reaction to Astarion's big reveal isn't terribly different from their reaction to Shadowheart's. Basically, "Oh, is that so? Well, leave me out of it." It's more negative where the reaction to Shadowheart's is more interest/fascination, but it's pretty low key given the circumstance.
So I think either it's a consistency that supports the argument that there aren't major problems with Shadowheart wrt Shar, or represents an overall casualness to dialog which could, in the most generous terms, be called immersion-breaking.
