Before answering this question, it must be noted that writing a book in that time was a task differing quite a bit from writing a book in the 21st century using modern computers, type-setting software, printers et cetera. To give an impression. Start writing a copy of Mark by hand yourself. ACER ASPIRE 7110-2369 REPLACEMENT LAPTOP LCD SCREEN

Were the last verses (of your handwritten copy) of Mark added later? I won't be that amazed if the answer was "Yes, of course!". An other question comes to mind: Did you copy the whole book before anybody else could read it?

It could easily be the case that others started writing copies of the book, even before Mark finished it. If we remember that copying by hand is such a tedious work, we can understand that it could be quite useful not to wait copying until the book was finished, but start writing the second, third etc. copy as soon as possible.

Were the last verses of Mark added later?
Yes

How could there be a short and a long version of the ending of Mark?
1. One possibility is that at first Mark wrote a short ending. Later on, he replaced it with the longer one, but some copies of the first version were already distributed.
2. Another is that someone wrote a copy of the book, but Mark had not finished the last chapter at that time, so the copyist wrote a small ending, awaiting the full ending by Mark himself.
3. Or Mark wrote no ending and several copyists added their own endings to the book
4. Or Mark wrote the short ending, and some copyist thought it usefull to add the longer one.

As you see, there are many possibilities, and I don't know which one is correct. What is important is to keep in mind that writing a book was a task largely different from writing a book now.