Does the Catholic church consider Mark 16:9-20 to be biblical canon? Many (but not all) scholars assert that Mark 16:9-20 was not present in the earliest manuscripts, and that instead it was added by later copyists. Does the Catholic Church consider these verses to be biblical canon?

Furthermore, is this a closed issue? If we happened to somehow find the very first copy of the gospel of Mark, and it either included or excluded these verses, would the Catholic Church be open to changing their stance?
At the Council of Trent, the Church officially declared as dogma the canon of Holy Scripture. This included &quot;the four Gospels, according to Matthew, Mark, Luke, and John.&quot; The council did not explicitly say whether the Longer Ending of the Gospel is canonical. However, it does declare that

if any one receive not, as sacred and canonical, the said books entire with all their parts, as they have been used to be read in the Catholic Church, and as they are contained in the old Latin vulgate edition; and knowingly and deliberately contemn the traditions aforesaid; let him be anathema.
(emphasis added)

The Longer Ending is in fact included in the Vulgate as well as in the New Vulgate. Further, in the declaration of the canon of Holy Scripture, the Council recalled that

our Lord Jesus Christ... commanded [the Gospels] to be preached by His Apostles to every creature,

a clear reference to Mark 16:15.
It thus appears that yes, the Church does consider the Longer Ending canon.