Feeling of unfairness is exactly the point of situation.
Real reward always was a flower they came for (together with saving the whole world ofc). As for the stake... On arrival Chief Pinu asks: "Tell me woman. How much are you willing to sacrifice?" and Claire answer is "Anything". John's answer is less meaningful as tribe's only interest is Claire, thus she's the stake.
Now a bit of speculations here: Actual trial of the Goddess is in the question "will this couple stay loyal to each other during the next 6 days?". It takes some time to fully corrupt Claire, but only one sex scene is enough for John to be considered disloyal. Losing challenges is just an instrument (stick) to get wife to cheat and put feeling of unfair game on John, while after winning Oluchi and Amara serve as yet another instrument (tasty looking carrot) to get John to cheat. Good mind game.
Yeah, you have a point. my gripe still stands, though, mostly because there are other elements, like a competitor in this very important wager who has nothing at stake (Ahanu). Also, I think the limit of losses before you irrevocably trigger bad ending, is like 5 losses, and past that point, even faithful runs get punished. All in all, I think this makes this game decent, art wise and content wise, but flawed in terms of premise. The unfair element in some NTR games elevate the tension and make it fun, but here, it also kinda makes it annoying, flawed as it is. Overall, though, I agree, good mind game.