I think that Joyce would have found out about Buffy's slayer abilities much sooner if Dawn was in the picture, because even in Real Me, Dawn states that it was "always" Buffy doing the slaying and getting the attention, and to me, the two years that Joyce actually knew about the slayer dosen't strike me as "always". I think it might have been easier then for Buffy to deal with Angelus, with the support and understanding of her family. I was always interested though, what did the scoobies think happened in the past from season 5 onward? Was it really altered that much in their mind?