wat info you talking about? The riot post?

Being a city state champ usually just means you have residency and live in a certain city. City state representative is determined by who is fighting on behalf of a city interests at any given moment. Chogoth could be considered a city state representative of Demascia if he was summoned to fight on Demascia behalf. Best example probably MF fighting on behalf of Noxus vs Ionia.

You don't need to have City states interest in mind to represent them.

I could see Poppy and Sona moving to a different City state and changing their City state status. Vayne is invested in Demascia, it's implied she isn't the only night hunter. She represents a secret group in the same way Leblanc does. Given how Demascia has strictest laws of all the City States, Vayne must have some strong political connections to get away with what she does.

Poppy's not going anywhere in the near future, it's required by her job to live in Demacia. Sona is a meh... As for Vayne, only thing we know about her is that she acts independently from the Demacian order, which we know for definite. She's probably like Mundo, she just doesn't get caught doing the things she does. If she has any connections at all, it'd be her nobility status.

Actually no, Zaun never allowed him to do as he pleased, I can find nothing in the JoJ or his background stated as such. In fact everything points to the contrary, where Zaun has to keep aquitting him of the 'abductions' because they couldn't find any evidence.

All I can find close to what you said is actually Noxus allowing Mundo to do as he pleases, and at best that is only a rumour and is not concrete.

It's more of an open secret I guess, everyone knows it's Mundo behind the 'disappearance' of people, but nobody can do anything about because efforts are all in vain, nobody can find any physical evidence linking Mundo to the disappearance.

Than it's Noxus, not Zaun, stop saying Zaun.