You want to know where the Jesus stories could have come from, if he didn't exist? First of all, there are very very few alleged eyewitnesses. Almost all the supporters he's documented as having had joined after his death, and relied on the stories they were told by those who'd allegedly seen him. Or on the stories they'd been told. If he was invented, very very few people need have been in on it.

Or none at all. I've seen one theory that the Gospels are based on a passion play about Julius Caesar that had been translated into Aramaic. We know there was a Caesar-worhipping cult around that time, and that they staged passion plays at the Ides of March; what if some Jews saw one of these plays, in Aramaic translation with all the names translated into contemporary Jewish ones (Caesar = Yeshua , Brutus = Yuda, etc.), and thought it was a true story that happened at Jerusalem rather than Rome?