The beheading was absent in GJohn because in the first gospel John was already dead and he was in Sheol, where Jesus descended at the incipit of the first gospel.
John asked to Jesus: are you the Jewish Messiah? The original answer of Jesus was removed by the winners but he didn't quote Isaiah. Then Jesus ascended on earth from Sheol and found the disciples of John, who abandoned their father Zebedee (=Dositheus) and followed him, believing him was John redivivus. The original crucifixion was the Transfiguration episode, when the same gospel ended with the disciples having fear, just as the women in our Mark.
John can be compared therefore in the first gospel to the OT prophets who considered Jesus as a tempter sent by the demiurge and rejected him:
“In addition to his blasphemy against God Himself, he advanced this also, truly speaking as with the mouth of the devil, and saying all things in direct opposition to the truth—that Cain, and those like him, and the Sodomites, and the Egyptians, and others like them, and, in fine, all the nations who walked in all sorts of abomination, were saved by the Lord, on His descending into Hades, and on their running unto Him, and that they welcomed Him into their kingdom.”
But the serpent which was in Marcion declared that Abel, and Enoch, and Noah, and those other righteous men who sprang from the patriarch Abraham, with all the prophets, and those who were pleasing to God, did not partake in salvation. For since these men, he says, knew that their God was constantly tempting them, so now they suspected that He was tempting them, and did not run to Jesus, or believe His announcement: and for this reason he declared that their souls remained in Hades.
From Book I, chap. 27,Against Heresies)