It is still debated that Mark was the first gospel. There is a growing school of thought that contends he wasn't. Secondly, each gospel writer tells the story to suit their theme. Because they bring other information to bear is not evidence of the evolution of the tale, but only possibly that they write with a purpose in mind.

Oh, and briefly, just about everybody thinks that Mk. 16.9-20 are a later addition, and most people don't give them any authority.

When we read the end of the gospels, we are seeing the story of the resurrection evolving within the Gospels, from empty tomb to Jesus hanging out with everyone for a few days. It's not a solid historical narrative, but an evolving legend. The earliest manuscripts of the Gospel of Mark only describe an empty tomb and a man in white (never identified) telling the women that Jesus is risen. The other gospels, then, expand on the legend and fill in more details to enhance the telling of it, and to try to make it seem like Jesus actually rose from the dead. Mark 16:9-20 seems to have been a later addition wherein Jesus explicitly appears to people. The later Gospels contain further additions to the story, but Mark (even with the later addition) doesn't have Jesus appearing to anyone who wasn't already his follower.