WolfishMusing, you are simply incorrect. There is no such thing as “an area within a karmelis that you can carry in”; I can’t imagine where you could have seen such a thing mentioned, since it doesn’t exist. The rabbonon forbade carrying in a karmelis, and did not make any exceptions. An eruv is by definition a reshus hayochid. That’s its entire point and purpose. You create a reshus hayochid in a karmelis or a reshus horabim and you can carry there. But midrabonon the reshus hayochid that you make in a reshus horabim must consist of more than just four strings on four poles, whereas in a karmelis that is sufficient.
