Derech Hamelech – A few points. What could have been a conspiracy? That the Rambam was chozer or that he wasn’t? Also, just to be pedantic, it couldn’t stretch back a thousand years as the Rambam hadn’t been born then. He lived slightly over 880-800 years ago, so who was making conspiracies about his deathbed confessions 200 years before his death?
Finally, what you write answers none of the questions I asked, especially not the one from his son (who, even if there was some sort of secret letter would have been one of the few to have seen it), it is simply an excuse once you already believe. Saying ‘maybe this, maybe that’ is all very well if you have a belief and need to reconcile it but to create the belief in the first place ‘possibly the letter was secret’ just isn’t convincing. The weight of evidence points to the Rambam not being chozer and if you choose to believe otherwise then that’s your prerogative but please don’t state your belief as fact (“even though he was chozer at the end of his life after he learned nistar”).