Quote you as an example but could go for anyone... Aren't both Anna and Trixie optionals? I mean, there's a path you can do only with Anna and one path you can do only with Trixie... if the spy talk with Chang appears in both cases, then it's hard one of those is the spy. I don't think the idea is any of them can be the spy depending on your choice. It would be a bit dissapointing to make it random in my opinion, like not something that was really thought while writing the characters, so I assume the spy is someone you can't avoid no matter the choices you make.
I always had both of them when I played (and I guess most of us did). Maybe someone that only had one or the other can tell us about the spy conversation with Chang, though I assume the theory of the spy would be interesting to have, just that those two are not suspects.
Sorry if using out of the game aspects to narrow down suspects is a turn off for some, I just can't avoid thinking about that too when deducting.