Pretty much what I'm asking is in-character-wise. How would the Yuan-ti(plural) go about doing this? Would they take temporary sponsorship from The Dark God, or would the grin and bear it, continuing to worship their Deity even with the loss of spell power/effectiveness. Pretty much what would the Yuan-Ti operating proceedure be in this situation.
Oh, okay. Right. Well, first of all I'm sure not all of them would be clerics. For those that were, it seems less likely to me that they would worship their deity in secret, with all that entails -- much better to do something else and then revert at the moment of greatest betrayal to others. Given that, there are two ways they could go about it:
1) Worship an ideal, to get around the whole thing.
2) Do what you said and worship the DG in name only, getting the spells. This seems very likely; the DG doesn't care about your innermost secret thoughts, as long as you aren't a member of his clergy. As long as you pay him lip service, he has more important things to do. Lots and lots of people pay him lip service. Then once he is overthrown (in the long-term goals of the YT), you can supplant him with the YT deity.
Aslo how would the city react to a YT Abomination slithering down the street, on his way to buy supplies(for example)? Guessing this isn't a big problem since Demons are allowed in the city, but always good not to make assumptions.
Most of my questions are more setting specfic fluff and logistics, to get a better idea of how to play such a character.
Well, they'd probably be freaked to shit, but if you had citizenship there isn't much they could do about it. Since they start off looking mostly human, shouldn't be hard to get citizenship.