The stories of the Resurrection and Ascension of Christ predate any of the written gospels. Remember it is the Jehovah's Witnesses who teach the formula that "beliefs are based on the Bible." This is not the way things actually happened.
Even the Christian Greek Scriptures indirectly prove that the Resurrection was well accepted and taught among Christians as a reality before Mark or any of the other gospels were composed. This is evidenced by the fact that the Pauline epistles were written before any of the gospels. If one accepts the general consensus that Mark was composed first among the gospel accounts, Paul's letters still came first by 10-20 years. Mark was written around A.D. 65-70 but Paul's first letters are from circa 50-55. Paul is already talking about the Resurrection of Christ in his epistles some 20 years before Mark wrote his book. Remember that Paul saw visions and received locutions from Christ who had already ascended to Heaven.
"The original ending of Mark does not attest to the Resurrection."
This is incorrect. While the oldest manuscripts end Mark at 16.8, the entire chapter is about Christ rising from the tomb. In Mark 1.4 we are told that the tomb was found open. In verses 5-7 a "young man" testifies to the women discovering the empty tomb that Jesus has risen and that they have been chosen to inform Peter and the other disciples about this because Jesus is going to appear to them in Galilee.
Even though the first Christians did not get their beliefs regarding the Resurrection of Christ from the written gospels (as Paul's writings prove they already had such a conviction even before he was converted), the earliest versions of Mark still end with claims that Jesus "has been raised."--Mark 1.6.
Noting that the added endings were later compositions, the Catholic Church still considered them part of the official inspired canon at the Council of Trent. Unlike Jehovah's Witnesses, "Christendom" views the finalized versions accepted as canon as the inspired texts, not the earliest texts that may be lacking some of the redactions that predate the official canonization process and declarations.