Very interesting premise. The thought that immediately occurs to me is what if the couple is no where near as "non-magical" as Regulus assumes? Loads of possibilities here ( farthest from out of left field possibility, the couple are Lily's parents - narrowly edging Hermione's parents or grandparents).
Author's response
Hello! Thanks for your thoughts :) Can you please comment on the first few chapters just as soon as I post them? And believe me I will; I just need to finish the prologue as it's proving quite frustrating to rewrite XP due to technical difficulties, I had to change the premise somewhat so that Kreacher had conflicting orders- One from Walburga Black, to keep an eye on her son as a Death Eater; and one from Regulus, who simply wished for death. To compromise, Regulus and Kreacher side Apparate out of the cave, and Kreacher goes back to Grimmauld Place to inform Walburga and Orion of their son's 'death'. The rewritten story will be up tomorrow :) I hope you enjoy it!
http://ficwad.com/story/224825 Here it is, enjoy!