Interesting, that the Floo would allow Lupin through. It would seem that Severus had some hope of his arrival - otherwise, if the Floo was available for anyone to come through, there could be complications of who-knows-who arriving. Very good for a challenge fic.
Author's Response: Thanks! I wasn't thinking that it was so much set to allow Lupin through as that there hadn't been time for Severus to seal it off from Grimmauld after coming through himself.