Wow. This is just amazing, and so very *true*. This is the first story I have ever read from Binns' point of view, and it just makes so much sense! I know I'm flailing, but the way you inhabit him is simply extraordinary. I couldn't help nodding like a bobble-head doll throughout this whole thing, as painful as it is to read at times. It seems you've really captured the way autistic people view and interact with the world, and it fits so well with what little we know of him from the books.
Your Walburga, too, is unexpectedly human, trapped in a world where prejudice is simply the air they breathe, and her *not* ending up the way she is would have been the more unbelievable outcome. The seemingly unlikely friendship the two of them make is as logical as Cuthbert says it is, and Walburga's unexpected act of kindness toward her friend is a lovely spot of light in a person we know falls all too deeply into Darkness in the future.
Finding Binns a position on the Hogwarts staff also gave him other friends, ones he could turn to when the inevitable happens. The cameos of Albus, Slughorn and Flitwick (I adore your Filius!) are such perfect captures of their respective personalities, and I love how they support him in their own ways when his friendship with Walburga comes to an end.
I never thought I'd feel sad for Walburga Black, but I can see how the class and gender constraints, along with Pureblood culture, combined to lock her into a situation where she must accept an unwanted marriage to a brute or become an outcast. I felt even worse for poor Cuthbert, who never thinks to try to extricate his friend from the violent situation in which they find themselves, nor to try to persuade her afterward that what happened to her was wrong and she doesn't have to accept it. Instead, the only thing he could think to do was to keep as far away from her as possible to prevent it from happening again, in the naive belief that his presence in her life was what had made Orion Black behave as he did, and that if he just went away, he'd make things safe for her. That really is heartbreaking.
Brava, Mystery Author - this is simply outstanding! This is definitely going in my 'missing canon' file, and I'll be rec-ing this later. :-)