I'm pretty sure that at some point it was supposed to part of the main SP campaign. I agree though, it was fairly obvious that it would be DLC as you were playing the game.
Seeing as this will drop at the end of November we'll get either one or two more between then and March. One is almost certainly the Citadel DLC. Whether that will be the last piece, to fill in the gaps or not though, who knows. It depends on what the other piece is I guess. If they've got something special up their sleeve, then that might be last. If it's just another piece of mid-game DLC then I'd imagine that the Citadel piece will be last.
Mike Gamble confirmed last night that you'll play Omega as Shepard. There had been speculation that you'd get to play as Aria.