Just rereading this story (as I really, really like it) and I kind of stumbled over this line: "young Draco will have both his father and his godfather pushing against the lessons Sirius is trying to teach him" - I assume that you are thinking of Snape as Draco's godfather? While Snape as Draco's godfather used to be pretty much an accepted fanon-fact before HBP came out, I don't think it makes sense now that we know Snape is a halfblood: what do Lucius and Snape have in common after all? They are both Death-eathers. That's it. Lucius is a rich aristocrat pureblood, Snape is not only a halfblood, he is poor and definitely from a lower class. Also through stuff that JKR told in interviews we know the approximate ages and Lucius is several years older than Snape. He was at best a sixth-year when Snape started school. They don't move in the same social circles, so it's highly unlikely for Snape to be Draco's godfather.
(Sorry if I'm being too nitpicky to your tastes but if I like a story, I really want everything to be perfect and logical, more logical actually than what JKR herself wrote. ;))
Think of it as part of the wooing process the Death Eaters used to lure Snape in. Fro the other side, why else would Snape go to such lengths to protect Draco, even risking his standing with Voldemort? Do we instead project all the fan-beliefs of Snape lusting for Lily onto Narcissa instead? Of course, maybe Snape has a thing for women named after flowers . . . h'mmmm :)