Joseph, I have no reason not to believe that Mark was reporting what actually happened. Mark's writing style does not lend itself to the poetic language that David uses in the Psalm. The language David used about still waters and green pastures is meant to invoke peace and protection. As I said before it is not a physical location. Jesus did not lead the people to the sea of Galilee, nor did he have the people lay down in the grass in a means to protect them. That was simply where they were when the event took place.
Mark clearly was writing what happened. Whenever he told a parable, he indicated that the parable was being told by Jesus. If this was a "parable" that was made up he could have easily had Jesus tell a parable about a few people being fed on a little food instead of reporting it as an actual event. The message he was conveying would have been the same.
To hold to your logic you will have to claim that all of the miracles explained by Mark are made up or copied from Old Testament. There is no basis to make this claim.
You are saying that Mark had no basis to truly know the events in Jesus' life but that he made up this person and all that he did from his imagination and Old testament scriptures.How would you explain all of the other aspects of Jesus' life explained by Mark- his travels, who he talked to, what he told the disciples, the trial by Pilate, the death, etc... Where would Mark's basis be to copy these events if your theory is to hold true? If it is his imagination don't forget that he would have to be smart enough to know all of the Messianic prophecies, he would not be able to contradict any of them with Jesus' life if he was to be believed to be the Messiah. You will have to invent a motive for this to be made up with the idea that Mark had something to gain. You will have to explain where he came up with the ideas that turned Jewish social structures on their head and explain why at the same time he was copying the old testament he was changing what the Jewish people had believed for centuries through this new made up Messiah. And you will have to explain why Mark would invent the exact opposite Messiah the the people were looking for, one that would be killed by Rome instead of overthrowing them. Then explain why the other disciples, Gospel writers and Paul would continue to make up stories about this made up Messiah and why they went to their deaths for Mark's fairy tale.
It is illogical and makes no sense.