The most likely scenario is that Paul invented (and marketed) Christianity just as Joseph Smith invented and popularized Mormonism. I personally think there was a Jesus, a charismatic itinerant who had gathered a modest following for himself but who suddenly died, leaving a disappointed group of followers. He might have even got whacked by jealous temple leaders. Along comes Paul the mystic, who convinces these zealous followers that he was in communication beyond the grave with Jesus. He might have even fabricated the story of being part of the conspiracy to "off" Jesus earlier but that Jesus had visited him post-death to get him to change his ways. It is clear that Paul knew little (if anything) about any details of the life of this preacher, and was careful to speak only of his personal communications post-death with him. He gains their trust and gets them to pool their money to fund him in mission trips to spread the gospel to other places.
I'm close to this but GMark is too much like reading
Old Man and the Sea or
Grapes of Wrath. There are many, many more similar works.
Authors are communicating with an audience. They stage events and have discourse in their writings that engage an audience. I wrote fiction for six years and there are no rules. At the time of GMark even plagiarism was acceptable. There were no patents or copyrights such as we have today.
And of course authors must write from experience, which does not mean they actually witnessed an event that they penned, merely that they read about or had told to them an account. The personalities in their works are amalgams of characteristics and experiences and events. They're allowed to be omniscient because it's their creation.
Today we can even find people writing seriously about the "historical" Paul Bunyan. But that isn't an accurate reflection on what they are doing because what they are writing about is the historical
inspiration for Paul Bunyan,
not the historical Paul Bunyan. It's a very real difference.