What a gorgeous story. You made me really care about Lucius and Narcissa in a way I never dreamed possible--you made them human. Cunning and vicious and amoral at times, yes, but fiercely devoted to each other. Human.
Ummm, what else....
God, poor misled Regulus--always pure, usually stupid, indeed. His grief over Kreacher, even if it could only be expressed as 'He is ours to abuse, thank you very much', was really moving.
Oh, and I should take a moment to shower confetti on this wonderful bit:
There was no mistaking the look of hauteur the boy gave Gryffindors' table, nor the pride in his walk as he came to join his rightful House. He took his seat next to Severus, offering his hand.
"You must be Snape. I'm Reg - and that unprintable censored over there is my brother. He's told me all about you - won't shut up, actually - just not your first name."
Severus' lips twitched into a fleeting smile, then a scowl in his tormentor's direction. "It's Severus, and if he's told you all about me, why are you shaking my hand?"
"Because I think everything he's told me is marvellous."
And how absolutely perfect that access to Snape's memories was through real, genuine remorse, something that he never got in life, I imagine.
I loved (because I am a sap) that the last part seemed quite hopeful, even though (or maybe especially since) Lucius seemed unsure if this was what he'd been trying to achieve. At least he could stop striving. "It did not bother him at all" is not a bad way to be, in the end.
Really a magnificent piece, and thank you for writing it.