That's interesting and completely plausible. That's kind of my point that we don't really know who Harry has been fighting and how they are related. Nemesis is pretty vaguely defined as a corrupting outsider influence, and we have no idea what rules it plays by. The Circle is some group Malvora, Madrigal, and Cowl talk about. Technically, it could just be made up by Cowl and not even exist (I don't buy that, but it's possible). The Black Council is just the name Harry came up with for a conspiracy between wizards he suspected were behind a lot of events.
Pretty much everything else about these entities is a guess, so there's a lot of fun here to be had in the guessing and challenging of the guesses.