Bob: I have an answer for those critics who claim that Mark is evidence that the Resurrection was a late idea cooked up by subsequent Evangelists. The usual claim is, because of the unique nature and uncertain authorship of the ending of Mark's Gospel, that originally the Resurrection wasn't part of his story. My reply: Then why does Jesus quite explicitly foretell his rising from the dead to the Apostles, and not once but multiple times, scattered all through Mark. Surely, I ask, you don't think all those passages are also late additions? And these are not isolated, stand alone verses, but woven into the very fabric of the Gospel, with long lead-in and subsequent narratives that hang off of those very predictions. You can't expunge the Resurrection narrative from Mark without doing violence to the entire work.
As to what the actual circumstances were behind the authorship of the ending of Mark, I doubt that we'll ever solve that one.