technically since we are messing with memories, it doesn’t really matter how he dies, just that he dies.  Besides every interaction that is centralized around Dawn is technically fake.  Okay, you caught me

I think that, due to the changes in Buffy's feelings toward spike, and your first chapter, it might make more sense to categorize this fic as what would have happened if Dawn was created a few years earlier, rather than these being the memories that were created with canon dawn. That is how i was reading it until i saw the above, and i think it makes much more sense, crazy time memory bending-wise.