Canon does not say whom Daphne Greengrass marries, or even whether she marries. On the other hand, Astoria Greengrass eventually marries Draco Malfoy; Astoria Malfoy dies of a congenital blood curse in 2019.
Canon does not say when the will of Sirius Black was read, or what the will's contents were. However, it is clear that Dumbledore's summary of the will's contents (in Half-Blood Prince Chapter 3) is incomplete.
Once Sirius Black dies, canon never mentions who the current Lord/Lady Black is.
Canon implies that the Black family owns other properties besides Number 12 Grimmauld Place, but these other properties are neither named nor located. Canon treats the townhouse at Number 12 Grimmauld Place as the main Black-family property.
According to the "1996" timeline at the Harry Potter Wiki (HARRYPOTTER dotFANDOMdotCOM slash WIKI slash 1996), Harry Potter completes his fifth year and returns home on the Hogwarts Express on 26th June, 1996. In Half-Blood Prince, Harry Potter does not appear in the story until Chapter 3, when Dumbledore visits Harry and the Dursleys at Number 4, Privet Drive. Dumbledore's visit (according to the Harry Potter Wiki's "1996" timeline) happens on 12th July, 1996. We are left to presume that in canon, nothing happens to Harry between 26th June and 12th July except for suffering a rotten life with the Dursleys—which, at this point in the heptalogy, is not worth mentioning. The reason I am explaining all this is that in this AU story, much of the story happens during the sixteen days between 26th June and 12th July.
Hello, Young Daphne!
Friday, 28th June, 1996
Two days after students left Hogwarts
Ten days after the death of Sirius Black
During dinner, Lord Cyrus Greengrass looked at his elder daughter and said, "Daphne, you are two years away from completing your schooling. Which means, you are two years from the time you may marry. It is time for me to begin negotiating your betrothal. Is there any young man you fancy?"
Black-haired beauty Daphne shot the man a look. "Father, you know that only Muggle-borns marry for love. I expect you will place me with whomever shall give the most benefit to House Greengrass."
Cyrus Greengrass asked carefully, "So you truly don't care whom you marry?"
Daphne flipped her shiny black hair over her shoulder. "As long as he isn't Draco Malfoy or Ronald Weasley, then yes, I don't care."
Daphne's younger sister Astoria said, "Daph, what do you have against Draco? He's smart, and definitely cute."
Cyrus was smiling. "No to Draco, and no to the youngest Weasley boy. Ha, I figured trying to set up a betrothal for the 'Ice Queen' would be harder than this."
Daphne scowled. "I hate that title, Father. And I do not understand why I've been given it."
"I do," said Astoria.
In Daphne's bedroom
Daphne had entered her bedroom to change clothes, before flooing over to Davis House to pay a visit to Tracey. But as soon as Daphne entered her bedroom, she got a feeling of disquiet, a feeling that something was not quite right.
After a minute, Daphne realised what was bothering her: a giant envelope lay on her pillow—an envelope which had not been on her pillow before dinner.
Daphne walked across her bedroom and picked up the envelope. She gasped when she read the front of the envelope—
No surprise, Daphne's name was written in the middle of the envelope, as the addressee; but her full name also was written in the corner, as the sender. The writing was her own, and yet it wasn't—the sender's handwriting was shaky.
Inside the giant envelope were several things, including a long parchment that had been pleats-folded, and that had Daphne's name written on top in a shaky version of Daphne's own handwriting. Daphne unfolded the long parchment.
Greetings, young Daphne, from a much, much older version of you!
Your professors at Hogwarts declare that the past can't be changed. Well, this letter makes an exception—but for this letter to go from my hand to your eyes requires goblin magic, plus magic from Harry Potter's house-elf Handy, plus special magic from elderly Harry Potter himself. As for elderly Harry Potter's contribution to the spell, he can do certain magic that no witch and no other wizard can do, either in my time or in yours. Young Harry Potter can't work this magic either, but this might soon change, elderly Harry Potter thinks.
You notice how vague I'm being about elderly Harry Potter's magic? That's because he has a Big Secret, and I've promised to keep his Big Secret as I write this letter.
But there is one truth about elderly Harry Potter that I tell you plainly, young Daphne: He is my husband. And marrying him was the best decision I ever made.
Harry Potter is the embodiment, the epitome, of what Gryffindors try to be. I don't mean this in a bad way—no, I mean this in a quite good way. Harry Potter, my husband, talks, acts and thinks like a solid-gold, capital-H Hero. Is it any wonder that I, who believed in my youth that only foolish girls married for love, am now so much in love with my husband?
At the moment, Daphne was strongly tempted to decide that the letter was a Weasley-twins prank. Except that inside the big envelope, besides a parchment that had been written in a shaky version of Daphne's own handwriting, was a second, smaller envelope that contained some kind of book inside. The smaller envelope was addressed to Harry Potter, with the sender being Harry Potter; the handwriting was different than elderly Daphne's handwriting.
(Daphne found in the big envelope one other thing: a recipe for a potion. The potion was oddly named, and the text of the potion-recipe was typeset-printed on Muggle paper.)
Daphne thought of the Harry Potter she knew: the boy with the messy black hair, the round glasses, the green eyes that showed so many expressions, and the Gryffindor robes. Daphne wondered, How could I ever marry him? Whilst Daphne did not love this Harry, she did like him, when the boy made a fool of Draco, Professor Snape or the headmaster.
Daphne read on.
Just about the time you read this letter, young Daphne, you'll have told Father, "I don't care whom I marry, just so long as it isn't Draco Malfoy or Ron Weasley." The good news is, you'll get your wish. The bad news is, the man I wound up marrying—I'll call him Sam Slytherin—was just as bad. I gave him two sons, so his House and the House of Greengrass both had heirs, then I spent my remaining years with the man concentrating on not killing him. I'm still amazed that I achieved this. When he died of a heart attack, I felt so free.
Meanwhile, Harry Potter killed Voldemort, a few years from now, then his married life turned strange. Potter and Granger never married—shocking, huh? Instead, Granger married Ron Weasley and Harry married Ginny Weasley. It was no surprise when Hermione spoke a divorce declaration against her husband in 2017, after Ron cheated on a Muggle driving test. (Don't ask me to explain why this got Hermione so angry; this is apparently a Muggle-born and Muggle-raised issue that Purebloods never will understand.) No, the only surprise about Hermione divorcing Ron Weasley was that it took her nineteen years to do it. Anyway, Granger clearly was hoping that Harry would divorce Ginny and would marry her—and Harry admitted to me that he was sorely tempted. But Ginny hadn't done anything to deserve a divorce, and a divorce would have saddened Harry's three kids as badly as a Dementor attack would have, so Harry disappointed Granger by never divorcing Ginny. Granger never remarried.
By the time Harry was free to remarry, because Ginny was dead, Granger had died a week earlier, of some Muggle disease. (Capricorn?) Two months after Granger and Ginny died, Harry asked me out; four months after our first date, Harry Potter and I married.
Six days from now, as I write this, I too will die. This explains why my handwriting is so shaky. How do I know the date of my death? Because Harry told me when my death would be, and I have reason to believe him.
So a week from now, if the timeline has not been changed by this letter, Harry Potter will have outlived Sam Slytherin, Granger, Ginny and me. Harry's long life is not due to luck or good breeding—no, it relates to his Big Secret that I am not telling you.
Daphne paused, and wondered whom she could tell about this letter. After some thought, she realised that if this was indeed a letter from her future self, she could not tell anyone in her family about it, without a strong reason—Daphne would be shipped off to Saint Mungo's if she talked too freely.
Likewise, Daphne dared not breathe a word to her friend Tracey.
The only person with whom Daphne felt free to talk about this strange letter was Harry Potter—whom Daphne barely knew.
Daphne read the end of the letter.
Enclosed with the letter to you, and with elderly Harry's letter to young Harry, is a recipe for a kind of healing potion. "Hemoglobin Repair Potion 46" is a strange name for a potion, but if Astoria drinks this potion at least once every six months, she won't die of the blood curse that kills her in 2019. Interestingly, this potion was invented by Holly Longbottom, the granddaughter of Hannah Abbott and Potions-class disaster Neville Longbottom.
The will-reading for Sirius Black was done a day or two ago, your time. Black's will declared Harry Potter to be an emancipated minor, after naming him the Heir to the Lordship of House Black. Thanks to Headmaster Dumbledore, who has an unhealthy urge to meddle with Harry's life, Harry never was told about the will-reading. (Never try to send Harry an owl; Dumbledore has put an owl-redirect on mail going to Harry, except for Harry's own owl.) Anyway, young Harry is now Lord Black and Lord Potter, but he doesn't know this, and it's up to you to send him an elf to tell him the news!
And then, young Daphne, ask young Harry if he would consider you as Lady Black, before some other Pureblood princess gets to him first. Such as, say, Susan Bones, she of the chest that boys notice.
As elderly Daphne had requested, young Daphne ordered a Greengrass house-elf, Bluegrass, to deliver to Harry his cross-time letter plus a copy of elderly Daphne's cross-time letter.
With the two letters, Daphne wrote a note for Harry: "I truly don't know what to think about any of this, except that I think the 'elderly Daphne' letter is authentic. I'd love to hear your thoughts. And congratulations on your elevation, Lord Black-Potter."
At Number 4, Privet Drive
Harry Potter was surprised when a house-elf whom Harry did not know, popped into Harry's bedroom and handed the boy a short parchment, a long parchment, and an envelope.
Harry was shocked when he recognised the handwriting outside the envelope as a variant of his own handwriting.
But that shock was nothing compared to the shock that Harry got, again and again, as he read the letter inside, which came in the form of a computer-printed, 73-page book.
But besides feeling shock as he read the book that his older self had written just for him, young Harry felt anguish. Whilst young Harry sometimes grew frustrated with Albus Dumbledore, Harry admired the headmaster; but elderly Harry argued that Dumbledore was a "dark lord" who was scheming to kill Harry as the means to kill Voldemort and to grab glory for himself. Young Harry strongly hoped that elderly Harry's opinions about the headmaster were wrong.