Exploration of a headcanon. Probably compliant with Book/Movieverse, up to the end of Deathly Hallows.
If you were to read the Daily Prophet as your primary news source, especially on those hectic days after the Second Wizarding War, then you could very easily come to the conclusion that almost every famous wizard alive was involved directly in the fighting in the Battle of Hogwarts. An understandable position, but one which ignores many key figures.
Take Garrick Ollivander, who possessed incredible intelligence, a keen eye… and a body that was frail and tortured. It was not as if he would not have fought, but the nonagenarian had the good sense to recognise that he would be a hindrance on the battlefield. Instead, he remained in Shell Cottage, with no clue as to the details of the ongoing battle.
That's not to say, of course, that he spent several hours pacing whilst looking out at the open sea. If there was one certainty, it was that at the end of the fight, there would be a substantial number of wizards and witches, wielding wands which were more likely to spontaneously combust than actually cast the charm it's owner desired. With the news that his shops in Diagon Alley and in Hogsmeade were in what one might politely call 'charred ruins', Shell Cottage would suffice as a temporary workshop. Indeed, it would have to, for the British wizarding world's only alternative would be Jimmy Kiddell, which was a disaster waiting to happen if he had ever heard of one.
And indeed, in the days after the war ended, word of mouth managed to rather adequately inform the wizarding populace that Ollivander was "Alive", "At Shell Cottage", and "Repairing Wands". Indeed, he was not particularly surprised to be woken up to see a queue had managed to form at the otherwise isolated cottage. Still, he was perfectly happy to collect the wands of the young Ogden (Oak, Dragon Heartstring, 10 and ½ inches, Rigid, burn marks indicating a miscast hex), the less young Belby (Hemlock, Unicorn Hair, 9 and ¼ inches, Sturdy, snapped tip from a shield charm that was hastily thrown up in the nick of time), and both Trigg siblings (There was a story there, but Ollivander was just glad they hadn't tried to remove the acromantula silk coating their wand themselves).
Still, as the wizarding world allowed itself to return to normalcy, or at least as close to it as such a world could get, he was anxious to return to Diagon Alley. Not only just to be able to work on rebuilding a stockpile of wands which had been plundered, but also to, frankly, get out of the hair of Bill and Fleur Weasley.
It appeared that all was going well. His little flat above the shop, whilst sparsely redecorated, was very much his again. The sign above the windows had a fresh coat of paint and the interior, whilst built with new wood, was carefully constructed to ensure that the layout and organisation would be identical to that which he had spent years getting used to. Sure, some of the scars in the shelves might no longer be visible, but it was very easy for Ollivander to let various memories flow back, fill in the gaps, reminisce about….
CRACK
Ollivander suddenly found himself the centre of attention, violently sweating and breathing heavily. It took a moment for him to realise exactly what had happened – he had, without intending to in the slightest, managed to apparate from his shop, right into the middle of the Leaky Cauldron, barely a hair-width away from bumping into a waitress, carrying an inordinate amount of food which thankfully did not fall to the floor. "Ah, whoops, apparently it's been so long that I misjudged it, I do apologise. Carry on!" Ollivander sheepishly whispered out. He was uncertain as to if anyone believed him, but none took the time to call him out on it if they didn't, so he made the short path back to his shop.
Standing outside his shopfront, he couldn't help but feel nervous. Still, he walked in as if nothing had happened, and for a few brief moments, all felt well. Then, just as he found himself thinking about how the shop used to look, it happened once more. With another crack, his scenery rapidly changed, heart beating heavy in his chest as he found himself in his living room, thankfully with nobody else able to see this time.
For a wizard, once was a coincidence, something that you simply live with. Twice? Twice was an active force working against you. That night, a scruffy brown owl carrying an envelope for the Auror's office flew out of his window, as he settled into a restless sleep.
As the sun rose again, Ollivander awoke to a sharp rapping on his door. Panic turned to a calm as he looked through his door's peephole. "Ah, Mr Potter, to what do I owe the pleasure?"
Harry – notably less skin and bones than he had been last time they had talked – responded calmly, "I believe you would know that", whilst holding up the letter Ollivander had sent just the night before.
"Ah, yes, well. I imagine it is some practical joke of sorts. I'm not sure exactly what it is, but if I spend more than a few moments in my shop, I find myself apparated a short distance away."
The pair descended down the staircase from the flat to the exit on Diagon Alley, before making the very short trip along the quiet morning street. Cautiously, Harry opened the door, before beginning a string of spells with "Homenum Revelio, Aparecium Incantatem..."
Ollivander, standing outside almost forgetting the situation at hand, noticed something which… couldn't be right. "Mr Potter, I am fairly sure that wand should not be functional, based on the last time I happened to see it.". Ollivander stepped into the shop, looking intently at the wand. "It's almost as in as good a condition as the day I..."
Ollivander didn't finish that sentence to Harry, who saw him suddenly apparate. A very fast spell-detection spell revealed that the only recent spells to trigger were his detection spells, followed by Ollivander's apparation. A tracing spell allowed Harry to swiftly join Ollivander, who had apparently decided upon the steps of Gringotts as his destination.
Scratching the back of his head, Harry relays to Ollivander that there is no external magic which would cause that within the shop, and that he needs to get back to the office. He also rather bluntly suggests that Ollivander checks in at St Mungo's. Not out of malice, of course, but apparently Auror training is relatively stressful, and occasionally it has been known to cause people to forget to filter their language. Ollivander understands – at least, understands enough to be thankful rather than put off.
Arriving at St Mungo's, Ollivander was glad to see the Inquiries queue was not excessive. The receptionist listened to him, sighed, and directed him to the second floor, 'ask for t' Involuntary Castin' ward' she informed him, in an excessively thick accent.
In the small ward on the second floor, Ollivander found himself talking to a tired Trainee Healer, who he barely recognised at first glance. Still, Malfoy listened intently, if not slightly puzzled by the situation described. Calling over the Healer-In-Charge, an elderly lady with scraggly grey hair, gave no better results, as she simply prescribed him bed-rest for several days, and to come back if the random apparations begin to expand beyond the shopfront.
It was, ultimately, Hermione Granger who figured out the actual problem, applying knowledge that a muggle-born would be more likely to pick up on. Sitting in Ollivander's flat a couple of days later, she explained over a steaming mug of tea that, "I suspect it's related to… events a few months ago? Your brain is probably protecting you from the memory, but the shop acts as a trigger, causing you to remember it. And then you're instinctively removing yourself from the situation – which in your case, is rather literal."
Ollivander mused this over his own mug, before asking, "In that case, what am I? Some sort of mental head case? How am I supposed to run a wand shop if the shop causes me to vanish?"
Hermione shook her head, "Broken? I would argue not. And I will see what I can do on that last point." With a completely-civilised-glug, she finished the tea, placing the mug down before apologising that she had to leave so soon. Still, Hermione had a mission, and once she arrived at the Ministry, she began writing out a letter.
Sitting in the warm Headmistress's Office, Professor McGonagall was poring over a collection of documents. It was almost a relief to see an owl rapping on her window. Doubly so when she saw the handwriting. And triply so when she read through the letter, absorbing the opportunity this presented.
For Hogwarts, as good as a school as it was, still could not teach a student with an uncooperative wand. She had seen many potentially great minds stumble, as either family heirloom wands were forced upon them, or as they couldn't afford any alternative to a second hand wand. Still, this would require some delicate negotiations.
At around 10 AM the next day, McGonagall appeared at the door of Ollivander's flat.
About ten minutes later, McGonagall was back en route to Hogwarts with a wry smile and a signed contract.
It was with some outrage amongst certain pure-bloods that the first-year letters that year, underneath the usual reminder as to broomsticks, included a new message.
"AN OLLIVANDER'S WAND WILL BE PROVIDED TO ALL FIRST YEAR STUDENTS. FIRST YEAR STUDENTS ARE NOT TO BRING THEIR OWN WAND."
Even with that said, and with rumours flying around, it was a relative surprise for those returning to Hogwarts to see that Ollivander was quite contently sitting at the teacher's table.
McGonagall was quick to bring students up to speed, for those who had not already realised. "After the events of the previous year, a substantial amount of the staff has returned to how it was in previous years. I am also pleased to announce the arrival of Professor Garrick Ollivander. He will be, as an optional course for third years and beyond, teaching Wandlore, in addition to providing the highest quality wands for Hogwarts first years."
Ollivander raised his hand as he was announced, in a very elated mood. After all, he had gotten a very good deal out of the matter. His son would run the Ollivander shop, in part selling stock that Garrick would make whilst at Hogwarts, and he could spend the rest of his days teaching his life passion, and witnessing young wizards learn using his wands, and watching how they develop. And beyond that, he now had the resources of Hogwarts, which meant access to more wand woods and cores than he had even at Ollivander's. And, the most vital part of course, Hogwarts did not bring back those painful memories which haunted him at his shops.
McGonagall continued with her speech "Since this is a very late introduction for the course to the school curriculum, those who are returning to Hogwarts for their Third Year will have the option to change or add to their optional courses over the first week. For those beginning their NEWTs, please contact Professor Ollivander as to course requirements if you'd likewise like to consider changing over. I believe, on that note, we can begin with the feast!"