This seems unreasonably hyperbolic; you are creating an artificial crisis. This is hardly the problem you are fabricating here.
Let's consider another case that does not involve deities. I create a character with a backstory pertaining to my family clan that I have also invented. A new player joins the game or a PC dies and a player re-rolls a new character. Having heard about my character's clan or watching me play, the player thinks it would be cool if their new character also came from this same clan that I created, possibly as a sibling or cousin. It makes for a great plot hook and easy way to seed the new character into the group. I am inventor of this clan and its history, status, etc. "(and thus, one assumes, the best authority) of the [clan they are trying to roleplay]."
IMHO, this common scenario is of a similar level as the deity above. I created the groundwork of what this deity represents. I established with the DM both its cultic orthodoxy and my idiomatic heterodoxy. If a new player joins, they are not somehow beholden to two GMs, as per your hyperbolic claim, but just one. I can inform the new player of my own understanding of the deity and its cult, but this is not dissimilar to having access to a GM and other sources. The player may consult both the DM and the setting materials. Or the new player may consult the DM and other players who are well-informed about religion or setting. Are other non-cleric players not as capable of telling a St. Cuthbert cleric player, "Hey, what you're doing does not represent the tenets of St. Cuthbert accurately"?
Or let's imagine another scenario. What happens when a setting-creator plays a game as a player in their own setting in but run by GM who is not them? E.g., Keith Baker in Eberron, Ed Greenwood in Forgotten Realms, or Gary Gygax in Greyhawk, Monte Cook in Numenera? Your imagined problem scenario basically precludes them from being players because now players are faced with "two GMs" or an inequality of the players: the GM and the author.