If I may to speak about rising from the dead it is next to impossible to really prove given that A) It's impossible as far as we know the action B) not many people were witness to seeing Christ after he died. The 12 apostles saw him and even one of them was in the same boat as most people who don't believe that is he wanted pure scientific imperical evidence to back it up. He wanted to see Christ, touch Christ, etc. But to still believe in the resurrection story cannot ultimately come from research it HAS to come through faith. The Bible in itself is a book about God spoken through the human condition using examples from history and experiences from people who encountered God that we (well The Church) felt were geniune experiences. I don't think I ever heard of the resurrection stories getting tacked on later. Luke's Gospel has the biggest section of the resurrection story and historians are convinced Luke wrote all of it and The Acts of the Apostles immediately afterwards. So I would need a source to this claim to read where they are coming from since from what I know of the history of the Bible is that, the letters of Paul come first, written around 40AD. The Gospels come later around 70AD with John being last who also wrote The Book of Revelations.
When it comes to what happened external around Christ, all of this is backed up through historical evidence. We are very familiar with what was happening, who were the major characters, political parties, situations, etc. What we don't have are documents that back up the Gospels, which in my mind would have been a bit strange if we did. This would be like finding a library of files about some random group of fisherman during The Anglo-Saxon war which someone would have to have written. So in reading Paul, if he tacked anything on it would be added human experiences from his perspective, much the same as an eyewitness might talk about an event from a matter of perspective. The Gospels are in the same boat. Again, different authors, two using outside source material we have still yet to find called Q, who simply wrote these books again bringing their experiences into the pages and also writing based on their own disciplinary style: Matthew writing as a teacher, Luke writing to a friend as if recounting events from a journal, John writing in a very stylistic prose that is miles ahead of the other three, obviously better educated in the art of writing.
What actually happened is written in these Gospels and the Letters. Whether you believe these people or not is a matter of faith. I seriously doubt that history can prove that Jesus did not walk on water or cure the sick, etc. All you have to back up these stories are the testimony of rather poor fisherman who have no political power whatsoever. Why it happened I don't think history can answer either.