Could there be ANY reason that a non-lizardmen like humans settled in lustira, or amaxons(for example) may sever ties with their original homeland and join forces with the lizardmen along with worshiping the old ones? Would they be accepted into their culture?
Interesting thought... I think that the lizardmen MIGHT accept the humans help- they are a creation of the old ones, and if they chose to worship the Lizardmen's gods, I think the reptiles could see them as useful to fulfilling the great plan. As for why the humans would defects... I think that it's perfectly reasonable- after all, its no different to defecting to chaos, only with the old ones, instead of khorn, slaanesh, nurgle, and tzeentch. It'd make a cool model, anyway .
In one of the older books there is a story of ....Marco Columbo I think he is called? a sailor / trader who comes to Lustria and ends up in a bind: he gets caught and is to be sacrificed. BUT! because of a serpent tatoo on him, the Lizzies think its a sign from Sotek to let him go, and he actually gets quite friendly with them. He even ends up having conversations with a Skink who has learned the human language. I think that story would give you a pretty good idea on how Lizardmen interact with humans. also, theres a novel I think describe something close to it: "The Burning Shore" hope it helps
I think just about anything that is clearly not blasphemous or evil could happen with alliances as long as the local lizards were convinced it could be part of the Great Plan. Humans returning to Old One worship, some vague plaque, some miracle or sign (would need to be significant to impress the lizards. All this could happen until the local Slann pays enough attention to go, "What the....?" Even then the Slann may tolerate the events, or just ponder them for a century or so. "Mustn't be too hasty now..." Turning to lizard worship might be a viable alternative to being sacrificed, eaten or set aflame. I assume the latter is what happens in the novel PhatMoFo mentions: " The Burning Shorts"
You got your stories mixed up. They are in the 5th edition Lizardmen book. A Cathayan military commander whose ship was hopelessly lost was captured and was about to be sacrifices when they saw his imperial dragon tattoo which the Skinks thought was an image of Sotek. The Lizardmen put him a holding cell while trying to figure out what the sign meant. He eventually escaped. Marco Colombo was a mercenary captain that set out to Lustria with the plan of trading with the Lizardmen. He offered up live Skaven prisoners and purchased LUstrian artifacts as a peace offering and then used his ships to fend off raiders and recover treasure in exchange for spices and pearls (the latter of which the Skinks thought were nigh worthless and were all to willing to give over). The Lizardmen Colombo talked to had a Skink who could speak Brettonian. I used that as a model in my fluff pieces. The Lizardmen don't have a lot of multi-lingual people but in mhy pieces their archives maintain records of what they do know and they make sure at least one scribe per generation can speak all the Prodigals languages. Also in the 5th edition book. An Araby explorer tried to barter with the Southlands Lizardmen to let caravans safely through the jungle, but the Lizardmen refused. They did negotiate an alliance to recover a mummified Slann stolen by the Tomb Kings. The Araby explorers knew where all the oases were which let the Lizardmen surive the desert journey. The humans got the Tomb Kings treasure and the Lizardmen got the Slann mummy. In both the last two stories I mentioned humans didn't really defect to the Lizardmen but they bartered with them peacefully. I imagine that is not TOO uncommon if language barriers can be broken. That's not the same as worship though. I read somewhere in offiicial GW fluff about Mazdamundi (or some other Slann) keeping lobotomized slave humans to try to find a way to use humans for the Old One's plan. In my own short story, Lord Merestar had some Skinks raise some humans from childhood and taught them about the Old Ones with the goal of creating a human Temple City that could serve as a base to evangelize to the Old World, which never made it to the evangelize stage due to sabotage from Chaos. In my other fluff piece, (SPOILER) a vampire and his minions are magically coerced into helping the Lizardmen. Coercion of all sorts could work. The Lizardmen are a Force of Order, but they are not gentle. They are inspired by Aztecs afterall. I am very slowly working on a fluff piece that covers big metaphysics in the Warhammer World. Without going into the nitty gritty details. The Old Ones intended for the creation of new gods such as Sigmar and the Lady of the Lake and all the elven deities to carry on their work so mortals who worship non-Chaotic powers ARE serving the Old ONes, albeit indirectly. Your OP is mostly about humans (or other Prodigals I guess) serving the Old Ones directly. It's plausible. Barrier One: Language. Few humans speak Saurian. Few of the First Children of the Old Ones (aka Lizardmen) speak the languages of the Prodigal races, but some do so this isn't insurmountable. Barrier Two: Aloofness. The biggest reason humans Fall to Chaos is that the Chaos gods do things RIGHT NOW. The Old Ones are the least proactive of all the Warhammer gods, so they basically need blind faith. Also, even if a human was willing and able to listen to an enthusiastic Skink priest talk about the Old Ones, the things a Skink and a human enjoy are so different it would be hard to see the appeal. A human wouldn't really see a whole lot of appeal sunning himself for hours and eating grubs. A Skink doesn't eat as much as a human, need functional clothes, and they are asexual beings. Humans and Lizardmen have less in common with each other than Humans and Dwarfs, huamns and Elves, even Humans and Skaven. That still isn't insurmountable. Daemons aren't very relatable either and most people would rather serve alien beings of goodness all else equal. Barrier Three: Mortality. Assuming you have a convert, that convert will grow old and die. To have a group of humans serving the Old Ones you need to have magically enhanced lifespans OR a breeding population teaching their beliefs to successive generations. Lustria and the Southland are rough places, so you'd need quite a few people to make as self sustaining group like this. You would either need a group under the direct patronage of a Lizardmen Temple City like in my aforementioned fluff piece or you would need a group of humans to return to the Old World and form a new cult (like the Knights Panther did with an Estalian goddess).