I don't think it's realistic for the backgrounds to be filled in game to any depth, you just have to approach this from a RP perspective: pretend the options that game presents you that are inconsistent with your background story don't exist.
If you're playing someone who doesn't consider themselves Baldurian, then just don't select that option when it comes up.
Having said that there are some obvious areas that need to be added for responses. If you're a priest of Shar you need extra options for SH, and if you're Gith you need some with LZ. Other key points will be relevant too.
But they need to just be the key ones, and related to the character creation options. Expecting to be able to add detailed backgrounds beyond this is unfeasable - just RP it.
