I always feel a bit guilty writing up my requests, because I'm implicitly asking for a lot of heavy lifting on the back story, and you've delivered in spades here. (Thank you, thank you!) I love the attention paid to Snape's genealogy (squib-ness as the source of shame driving his family dynamic and the reason for his odd social status as a student) and the world building that leads up to Yaxley's awkward proposition (sex? alliance? I'm not even sure he knows what he wants, exactly). And of course his murder of Dumbledore changed everything for him among the Death Eaters--tying that individual action to debates on "blood quotient" makes perfect sense.
I also appreciate the slightly creepy attention paid to physiognomy here (pure bloods would think that way, wouldn't they?), and the way you've jumped right into the mess that is JKR's vision of hair color as the portal to a person's soul and tried to make some sense of it. (Part of the reason we knew Snape wasn't going to be a fundamentally bad character, ridiculously, was the way his hair color matched Harry's and James' and Sirius.)
Snape was fabulous--
Damn Dumbledore and his stupidity, getting himself poisoned and having to abandon the fight. Having to have Severus kill him.
--insightful, resentful, self-centered, essentially pragmatic. (I loved that line.) And the way you developed Yaxley was fascinating, too--a bit like Lucius Malfoy and a bit not.