I think the problem with leaving most of the Shar information to in-game books and such is that it's pretty likely that the player just...won't retain the information even if they did read it. There's a bunch of books around the game, all with all kinds of info, most of it unrelated to the main story. Why should the players do more than think "oh, this story is neat" and just file it away in the back of their minds never to be touched again? I'm all for expecting more of players, but for new players, context needs to be given. Regardless of Larian's intepretation of Shar, they should give us a concrete sense of who Shar is in the setting so that we can properly roleplay our characters. It's as simple as this; we should know what our characters know. If our character is expected to know who Shar is, and the dialogue options for the reveal imply that they do know, then we should know as players. ESPECIALLY because this is tied to the story of a major character. Larian is still telling a story, and if they want this reveal to land, then they should make sure we have the information needed to give this reveal an emotional context. This is a story, you do actually have to give players clear information sometimes so they know what's important and what to care about.
And just to touch on whether or not Shadowheart's armor is too obvious, I think the problem is that we know explicitly she's a cleric, it's something anyone can see. Sure, maybe if she were a fighter or a rogue or an average person, people coudl brush off the disks as jewellry. But Shadowheart is a cleric, it's observable that she's a cleric, someone who is, by definition, deeply devoted to a deity. So we take that given information, add that to the fact she's decked out in a particular symbol, then it's only logical to assume, even if you don't know about Shar, that the black disk is related to the deity she serves. If I saw someone who prominently wore a bunch of red squares as jewerly, I would assume they just liked the look. But if that same person told me that they're deeply religious, I might start to assume that those squares are related to their religion somehow. And the fact we can't ask Shadowheart what deity she worships is weird in that regard. Even if she lied, that'd make sense and be fine. But the story as it is gives us no choice but to not be interested.
