A And very early on the first day of the week they went to the tomb when the sun had risen....so, with cheerful disregard for good methodology, I leapt over to the Gospel of Peter to borrow the last line. It doesn't really work -- there's no transition there between B' and A', and the C' and B' brackets are doublets that suggest they originally might have been the center of the chiasm -- but while I was rooting around in Gospel of Peter, I realized that if Mark had originally written an ending that resembled GPeter, the legend of his connection to Mark might have its roots there. Here's the last section of GPeter:
B And they were saying to one another, "Who will roll away the stone for us from the door of the tomb?"
C And looking up, they saw that the stone was rolled back; -- it was very large.
D And entering the tomb, they saw a young man sitting on the right side, dressed in a white robe; and they were amazed.
D And he said to them, "Do not be amazed; you seek Jesus of Nazareth, who was crucified. He has risen, he is not here; see the place where they laid him. But go, tell his disciples and Peter that he is going before you to Galilee; there you will see him, as he told you."
C And they went out and fled from the tomb; for trembling and astonishment had come upon them;
B and they said nothing to any one, for they were afraid.
A It was the last day of the feast of the unleavened bread and many people were going out, returning to their houses since the festival was over.
"(58) It was the last day of the feast of the unleavened bread and many people were going out, returning to their houses since the festival was over. (59) But we, the twelve disciples of the Lord, were weeping and grieving, and although everyone was mourning because of what had happened, each departed for his own house. (60) But I, Simon Peter, and my brother Andrew took our nets and went out to the sea. And with us was Levi, the son of Alphaeus, whom the Lord [. . .]"
Imagine if early versions of Mark had a sudden shift to the first person at that point, with Peter as the narrator (GPeter almost certainly knows the Synoptics). Then the legend of Mark's connection to Peter originated as an etiological myth to explain why the "I" suddenly cropped out in the now-vanished ending.
Just a suggestion.