How on earth did these fans come up with this theory? Good grief!!
On the scale of things that's probably one of the least bonkers Snoak theories I've seen in the past 10 months. Other range from "it's old Ezra!" to "it's a clone of Palpatine!"
The only one that seems to have legs is that he's some ancient being that predates the Rule of Two. Maybe even Darth Bane himself. The only clue to this is a scene in 'Life Debt' that reveals Palpatine was very interested in excavating something very old buried under the sands of Jakku. So it's still very much open to speculation.
I could have sworn there were an EU novel or two that featured Obi-Wan in an adventure away from Tatooine.
As mentioned previously, there was a pretty damn good novel about his first few months on Tatooine that sadly came out right before the EU cut-off. I think there were also some comics here and there, though nothing you'd really want to build a film around. As much as I did enjoy the novel, I'd have a hard time seeing them turn something like that into an exciting holiday movie. Not unless they're really willing to stretch the kind of genres they're willing to tackle with these "Star Wars Stories".
There is however an ongoing canon storyline in the current ongoing Marvel comic that depicts excepts from his diary, though it's mostly set much later when Luke is about 9 or 10 years old. So far he seems to have been planet bound the whole time. The recent Ahsoka novel also has a very brief scene that features him communing with Qui Gon on Tatooine (presumably about a year after RotS.)
There were, but the majority wouldn't know that, thus would expect him to have stayed put, thus "everyone knows".
I don't feel like the audience assumption that he'd stay is a big a stumbling block as the idea that it'd take something pretty damn major to get him to abandon his post, even for a short time. Since he's committed to protecting Luke and forsaking any sense of responsibility for anything that's happening in the galaxy, it would probably have to be something that directly threatens the boy.
One of the recurring themes of both the novel and more recent comic stories is his struggle with ceasing to be a Jedi. To force himself to not get involved, even in the face of obvious suffering and tyranny. Both in the wider galaxy and right at home on Tatooine.
That's probably your main hook for the story. The reluctant old warrior with a guilty conscience.
