I'm wondering if this has to be a case of calling time or if instead you could adjust your expectations of the friendship instead? For example knowing that talking for hours can't happen now but instead you will have a pleasant time as you describe.

It sounds like you care about her greatly to come on here to try to understand what she is going through better, and while that's a positive thing to do it may be that understanding her condition is not realistic. It might be more a case of accepting her as she is and accepting you may never understand her.

Only you will know whether this friendship is ultimately viable however.