Thanks PP for the Thread, and particularly your post above, that is a very clearly and well expressed summation of what happened.
We of course cannot know what was in the minds of the Gospel Writers, but I think your analogy of the "Left Behind" Books is a good one, and really fits what I imagine the Writer of "Mark" did, he wrote a Novel, then "Luke" and "Matthew" added to it, perhaps in their minds their Works being more the writing of a Gospel.