The mystery is resolved. Johann Lorenz von Mosheim was the first to argue that the Didache or Teaching of 12 Apostles was originally a book of instructions addressed by the Jewish High Priest to the synagogues of the Diaspora. The 12 Apostles meant in the title of the text were the 12 Servants of the High Priest who were working in the Diaspora in the his name by carrying directives etc from/to Jerusalem. When the Jewish Christians co-opted the text and christianized it, there was born the need of 12 Christian Apostles sent from the new "temple" and from a new "high priest".
Do 2 + 2:
The original Jewish 12 Apostles worked in the Diaspora.
They became christianized as 12 Apostles of Jesus Christ.
What allegorizes the Diaspora in Mark?
The Galilee of Gentiles.
Hence, the idea of a Galilean Christianity is purely a myth.
Independently from the historicity of Jesus.
Nihil enim in speciem fallacius est quam prava religio. -Liv. xxxix. 16.
Hence when Peter denies that he is Galilean, he is denying de facto that he is an Apostle of Christ in the Diaspora.
And when the Risen Christ appears only in Galilee, "Mark" (author) is denying that the Jewish Christians of Jerusalem knew about the death and resurrection of Jesus, unless they go to "Galilee", too, i.e. by joining with the Paulines.
Nihil enim in speciem fallacius est quam prava religio. -Liv. xxxix. 16.
