While at Southeastern I was fortunate enough to get to attend a conference on the last twelve verses in Mark. Speakers included Daniel Wallace, David Alan Black, Maurice Robinson, J.K. Elliott, and Darrell Bock as the "moderator". Fortunately, B&H Academic has published their papers in a short book titled Perspectives on the Ending of Mark: Four Views.
I won't go into great detail about the papers, but I will say that I find a great deal of Elliott's argument very persuasive as well as Dr. Black's and Dr. Robinson's. That is to say, I suppose, the only argument I didn't find persuasive is Dr. Wallace's. Dr Wallace argues that the ending at 16:8 with the postpositive γαρ is original. Elliot notes that if this were some kind of clever device on Mark's part, a cliff-hanger, then it was entirely too clever for Mark's immediate audience. Already by Justin Martyr's time the longer ending seems to be known. Robinson also cites Larry Hurtado who shares the belief that if Mark ended with a γαρ it was so clever that nobody got it until modern scholarship came along.
I think a conglomeration of the three views I find persuasive could work. One thing I'm thinking about is Elliott's proposal that a leaf was lost (a leaf perhaps containg Mk 1:1-3 as well, though Elliott admits the leaves could have been lost at different times). Lets say that Mark published a Gospel that did not end at 16:8 and then due to either scribal error (which Elliott finds unlikely) or due to a leaf being lost, the LE found its way into the manuscript witness. It would have to have happened extremely early, I think, in order for both Justin Martyr and Origen to know of the LE. The manuscript(s) containing the LE would have had to have found their way, or served as exemplars for manuscripts which found their way, into both Judea (where Justin was) and Alexandria (where Origen was) within a very short amount of time.
Another issue I thought about is the Patristic witness of the LE. Unless one of the Fathers wrote a commentary or worked their way through a lectionary, how would you know that a Church Father only knew up to 16:8?