I think the idea of a cult leader in the Trekverse is not bad (though done badly in Way to Eden). The way Sybok takes over the ship is original, but making people feel better about their mistakes - which is typical cult stuff, but this time done through telepathy. Its actually clever.
And Sybok as Spocks bro also solves for continuity problems, even if it made Gene mad. Sarek MUST have been betrothed to a Vulcan woman from birth just like Spock. Something had to happen to her to allow Sarek a choice of bride. This makes as much sense as any.