Omya,
In discussing Mark's Gospel, let's spend more time discussing why the Gospel is included in the Holy Bible, despite the controversy over it's ending, which you correctly point out, is under dispute as to whether it was in the original.
1. On two points the tradition of the early church is unanimous.. Mark's Gospel was written by John Mark and presents the presents of Peter. Papias in AD 140 is quoted by Eusebius, one of the early church fathers, as saying "And John (the apostle) also said this, Marke being the interpreter of Peter, whatosever he recorded, he wrote with great accuracy... he was in company with Peter, who gave such instruction as was necessary. (Eccl. Hist.. 39)
Justin Martyr another of the early church fathers wrote in AD150 in support of the gospel, it's authorship and it's basis on Peter's teaching, as did Iraneous in AD 185, Tertullian in AD 200 and Origen in AD 230.
it should be pointed out that the authorship of this third Gospel is more universally accepted than any of the other three Gospels.
Most people put the book written around AD 65 and AD 70, so some of the original Disciples such as John were still alive and had yet not been martyred.
The two oldest manuscripts in Greek the Vaticanus and the Sinaiticus (both from the fourth century) do omit the end of the Gospel after 16:8 and most scholars do believe it may have not been in the originals, however this is far from conclusive. The early church fathers who prescribed which books should make the canon of scripture certainly included it and I will keep looking as to why it was accepted, unless someone else finds that answer for you first.
Most scholars accept that Mark's Gospel was written first and that Matthew and Luke both used Mark's Gospel when they composed their own. In fact 95% of Mark is found in either Matthew and or Mark's Gospel. Acts 1:22, 2:22-24 and vs 36 all ascribe to the importance of Mark giving us the basic message of the primitive church. The theological and the history in this Gospel is widely appreciated and attests to why the entire Gospel is included in the canon of scripture even today, when the last 12 verses are under suspicion as not being in the original.
Hope this is useful for you
Carol