The path of the world is no sure thing. Events are not certain, though fools and soothsayers say otherwise. Even that which has already occurred means nothing! Nothing to a witch or wizard with sufficient determination. On one night, for example, a boy and girl unwound time’s spool, saved more than one life and discovered, by undoing a darker timeline, a power strong enough to drive darkest evil away.
So give us a cruel world, and we know it may be possible to wind back until we are in a better one.
Suppose Mr. Malfoy, who would unwittingly bring the war to a head and discover a certain cabinet and change the fate of an entire school — suppose his father triumphed? And sent him away to a different place, a school much harder and colder, where he became by degrees a thinner and overall less important figure — a footnote, really — simply the boy cheering on Krum in a throwaway scene?
Then no one would invade the school at all, not for thirty more years, and in all that time the war would rage on, would progress to the continent, would claim many more lives.
And what of Ms. Lovegood? Suppose her mother, that stabilizing force, had never died. Suppose she were there to counter Xenophilius’s every curious mood, suppose she were there to provide loving support and careful advice to her daughter, to furnish the secrets of friendship.
Why, then Ms. Lovegood will be close to young Brocklehursts and Bodes, if still considered a bit odd; and when some grim young hero rushes past her, looking for the secrets of a certain ghost, she will not have the answer for him. She will have been busy with friends, not lonely enough to have sought chilling and ghostly answers.
Suppose Mr. Weasley worked very late, all through until the morning, and never came home to his loving wife on a cold night, and in eleven years’ time been short one son? Or perhaps it happened a year later, and they were short a daughter?
Suppose that two dentists had moved to Australia long ago? A long-desired trip! And there their little girl was accepted to some curious outback school, where the students kept massive spiders as pets and played rougher games than Quidditch. She thrived, of course. Books and cleverness. But in the meantime Britain was lost, and two Hogwarts schoolboys who need her lost with it.
Suppose Mr. Longbottom’s parents had ceded a certain case to Auror Shacklebolt, and let him meet certain doom, while they accepted Uncle Algernon’s advice to remove to Canada? Suppose that, when the children of Hogwarts looked about for someone to remind them what courage was, courage was not there at all. He was in Vancouver, safe and warm and loved, and they were terrified, and soon to give up.
Well. Events are not sure things. Though we might suppose none of this happened, suppose something like it did. Suppose Harry Potter faced certain doom aged and friendless and alone, and suppose that instead he unwound the spool of time somehow, undid his whole universe. And in the undoing he found these strange crossroads, these moments where such crucial people, and others, just as crucial — a professor who might have married her Muggle, a godfather who might have chosen his family, and so on — set in motion different events.
And each time, with every change, he found an unwinnable future, something unspeakably dreadful, as though in order to produce a world where enough might live on and the world might continue to spin, he would have to knit the world into the right pattern, to find the right alchemical mixture of events.
Suppose, having undone the wrong at the heart of his universe, he set upon a novel idea: to fix all the others. To travel to one where a golden-haired girl was living locked up in St. Mungo’s, and her brother very merrily conquering the world, never once dispensing a lemon drop. That had to be fixed. And so too the world where a young man never lent away his Invisibility Cloak, and was able to shield himself and his wife from death, and their son grew slightly spoiled, and much less malleable, and did not succumb to the would-be conqueror’s machinations, and all was lost.
And so too the world where Peter Pettigrew did the right thing.
Suppose that in this timeline, the one recorded, the real one (or so we think), whenever Hermione needed the right book, there was a strange attendant in Flourish and Blotts pointing her to it, for without its knowledge she might never have accomplished what she did? Suppose there was someone to come across a broken De-Luminator in the Headmaster’s possession and to fix it for him, so that it might be handed off to Ron Weasley someday? Suppose it was him. A sufficiently determined Harry Potter.
Would he have left some clue behind, some key for how to do it without causing too much havoc? If it had been Hermione, she would have. Suppose it was Hermione. In one timeline, perhaps it was; and she wrote a book, detailed and instructive, and very clear if one were clever enough to decipher it. Or suppose Ron did, presenting it all like a game of chess.
Suppose the book fell into the right hands. Suppose it fell into the wrong ones.
They would only need to change events to achieve their ends. This is easy, for a magical person with enough determination. So perhaps they did just that.