To me the idea that fits the facts while conforming with the orthodox story to the extent that the orthodox story had to have some truth in it, is this:-
The Jerusalem people "St Paul" mentions, were Jewish mystics, "disappointed Apocalyptics", as per Ehrman''s description of such types. (For Ehrman, it's disappointed apocalyptics who form the core of the development of Jewish Gnosticism. One side-effect of this is the prevalence of apocalyptic language and terms in the Christian canon, but by and large they are metaphorical hangovers, habits of usage from their older, apocalpytic precursor belief system.)
The earliest "layer" of Christian thought in its Jewish form is from a period of optimism following Caligula's death after his threatening the destruction of the Temple unless the Jews would put up a statue of him as a god in it. (Note: Revelations is from this period, it's actually not retrospective, but comes from a period of propaganda when Jewish mystics were warning their fellow Jews about what would happen if they agreed to allow the Romans to set up "the Abomination of Desolation" - Caligula's statue as a god - in the Temple.)
Caligula's death was seen as a righteous kind of victory, as evidence of the start of the Messiah's campaign, the herald of a new dawn, a new Aeon. Finally a victory against the Roman bastards! Our god struck him down!
This is what suffuses the earliest Christian materials with a sense of a victory won. i.e., in the earliest Christian layer (some of the Epistles), there is a prevailing tone of a victory having been already won, already done and dusted - that being what gives the cathartic emotional relief-power of a new, burgeoning religion.
"The Messiah has already been" is the core message of these earliest Christian mystics. In contrast to other Messianists, who were either looking with longing to a future Messiah claimant, or putting their bets on some contemporary putative claimant to Messiahship, these disappointed-apocalyptics-verging-on-gnostics thought they found evidence in Scripture and in recent events (Caligula's timely death) that the Messiah had already been and had by his obscure crucifixion at the hands of the Archon (the Rulers of this world, referring to the demons in charge and their human puppets), secretly won a spiritual victory, thereby fooling said Archons, who had been lying in wait for the expected royal man of action. They expected the effects of a supposedly historical, but deliberately obscure spiritual sacrificial act to sort of "crystallize" through into a progressive worldly realization of Jewish hegemony and world peace.
"St Paul" is someone who may have been initially hostile to this cult, but later converted (or the entire conversion story might be bollocks); at any rate, while he participated in a similar perspective on religious truth (that victory had already been won, spiritually), he viewed the victory as more universal in scope - not just for Jews, but for all mankind.
And then ... and then the shit hits the fan. Then you have the two Jewish Revolts and the Diaspora.
After this dislocation, there are scattered remnants of the spiritual circles seeded by "Paul", the true origins of the movement are obscured - partly out of sheer dislocation of social ties, partly by charlatanry (e.g. Polycarp, perhaps).
The remnants in Rome and Alexandria then either concoct or discover a novel concept - they invent the idea of an apostolic succession of people who actually knew the cult figure, the Messiah, in person. Perhaps GMark is a literary novel, part exemplary (Stoic-type) biography, part complaint that the Jews hadn't listened to the cult's Messiah when he was around.
When was he around? Well, GMark invents a more specific time and place for what was previously a merely fuzzy "some time ago" in the very earliest materials.
Bear in mind that this entity is totally imaginary, all the way through. He's an imagined cause behind recent goings on that had been favourable to the Jews, who had been prophesied. But while many of these cultists had met and probably spoken to Him in visionary experience, none of them had met, or even claimed to have met, their Messiah in person.
All that happens is that later on, the notion is invented that the earliest cultists knew the Messiah in person.