First, yes, pay is based on who the staffer works for. The congress person decides the amount, it is not a set pay scale. They can decide how big their staff is, or augment it with unpaid interns, which many do. AOC is one of the first (and might be the only) to pay her interns, because she recognized that only those from wealthy families could afford to live in Washington without pay. The specific budget she gets is from the government is hers to do as she pleases. Now, there are maximums that they can pay a person, but as far as I can find, there is no basement.
As for the Met Gala, the point you fail to grasp is that while optically it was bad, the reality is she was cleared by the ethics committee. As much as you would love to argue differently, that is just a fact.