Concerning the Hagravens and Briarhearts, I have to wonder what their old ways and old magic refer to, considering that the oldest magic (Magnus-based) would be the Psijic Order. The same goes with the "old gods". I hesitate to say that it's contrivance from the writers, but as far as the et'Ada are concerned, there are no older gods, at least not the animistic ones the Forsworn seem to worship.
I always took it as
their old ways,
their old gods, not necessarily the most truly old ways and gods of men. For all we know, their old gods are simply the daedra in funny masks, and, hell, they may have originated in the Third Era for all we know. People do have the tendency to look back to their grandfathers and just go, "well, that's how it's always been."