I am so sorry that this happened! I do have to say though that I agree with Chigirl--the store is reputable. I think that there are different kinds of damages and that if the leather were damaged, a repair would be unacceptable (to me--I can only speak for myself). A missing rivet (or whatever they're called) though, is a damage of a different level. If it is a regular Balenciaga rivet, exactly as the others, then the bag becomes whole again, which couldn't happen with damaged leather. I think I would want a warranty on their not falling off again, though! (But, I would expect that anyways, with any bag purchased from them, even one that arrived whole.) I can understand their reluctance to shift to an alternate color. Yes, they probably should have obtained another Origan from Balenciaga and made the substitution and taken the hit--but, the Origan they are returning to you is not damaged in any real sense, once the "rivet" is replaced IMO.
Buying a high end bag shouldn't lead to grief. I do hope this gets resolved in some way that leaves you feeling better, Kaka.