One thing you are correct in, each Gospel author built upon the elements of myth of his predecessor(s). Once an element is accepted as part of the story, rather than drop it if it conflicts with your theology, you flip it to match your theology. This is a spot on observation! But your argument falters after that. (I'll accept that first observation independent of your conclusions, no need to throw the baby out with the bathwater)
[digression]
This process is what I refer to as the polemic debate developed at the dinner table (the Christian ritual of eating the meal or "breaking bread"). One can see how Marcion while having a Docetic Christ, had him eat fish to prove his resurrection was real. This became the argument for a flesh Jesus. Then Apelles (and other Gnostics) sought to explain this flesh of Jesus by saying he borrowed elements from the heavens as he passed down from the Pleroma to earth. You can see a similar development with Adoptionist positions, and also the proto-Orthodox until we eventually arrive at the Trinity.
[/digression]
As for the robbers, I am not sure they are part of the "first version" of John which I refer. I think only these word were original from verses 19:17-18
19.17 So they took Jesus, and he went out, bearing his own cross.
This is to reiterate that John's version of Jesus never asked for the cup to be removed (he refutes the synoptic accounts), and that no Simon or anyone else bore the cross for him, he alone took it up. The mention of Golgatha and the robbers is purely a later harmonization to the synoptic accounts, as neither plays a role in John's story. In fact they intrude on the story, which continues about the cross, and make the crucifixion occur before even writes the inscription!
Now if you wish to say the Catholic version of John opposes the Marcionites, well I think you have your work cut out for you before you can even make such a statement. You have to identify what is later Catholic revision, and then identify whether it has any specific theological value or is simply harmonization. My take on the ending is that it went something like this:
18:29-19:23a (stops before "But the tunic was without seam, woven from top to bottom"), then 19:28-29 read,
"After this Jesus, knowing that all was now finished, said, "It is finished"; and he bowed his head and gave up his spirit.
Then 19:38-20:1, 20:11-20:15 and then some parts of 20:16-18.
I am partly convinced Peter does not appear in the original version of John. (note his absence from the walking on water, and in general the secondary nature of everything he says -- you can remove his every verse from the story and it flows seamlessly.)
Anyway, until you reconstruct that first version of John (and in my view its roughly 90-95% of the content of chapters 1-19, with surprisingly little added to Catholicize it; and ironically I think the first published version was itself two layers -- there is a subtle difference in theology between the narrators words and the story in that version).
“’That was excellently observed’, say I, when I read a passage in an author, where his opinion agrees with mine. When we differ, there I pronounce him to be mistaken.” - Jonathan Swift