Me.. I would probably keep up the cause. One what happens when that Mayor is no longer Mayor? I don't know what the fine is if any, other then just getting rid of the chickens. But I wouldn't want to pay one or have any of it go to court. I feel since Des Moines and many surrounding communities let you keep chickens in town, it could be a easy win. Maybe this lady that contacted you would be willing to talk at the city coucill meetings and you could go for support? Since she knows the mayor maybe she will have more of a shoe in for you.
This is just my opinion, but I'm a chicken. Maybe keep one or two in the basement in the meantime if the process goes on a long time before you get any answers or the law changed? I'm not sure how smelly two chickens inside would be though. Have you seen how this lady has her chickens? 18 seems like an awful lot in town. I'm just picturing my yard and it's rather small. LOL
Good Luck with whatever you decide. I will keep checking here see what happens and what you decide.