Nearly all of my Shar knowledge is from BG I and II and I'm still in the dark...about most of what's happened pertinent to BG:III.
You might say I'm the perfect test subject for this.
I think the big issue is that Shar caused the spellplague by getting cyric to murder Mystra about 100 years before the game is set. The spellplague was a cataclysmic event that killed millions of people, destroyed cities, etc. Baldur's Gate wasn't effected much, but most of the rest of the world was.
Maybe there should be a text during the loading screen about this. Maybe your character can make a Religion check after she mentions Shar, and it fills your character in a bit.
