If that is the case, it sure sounds like AK is the proverbial Office Manager who rules the stationery cupboard with an iron fist. lol.
You are keen to denigrate Angela Kelly, aren't you Mummy195.
If you had read her books, you would have discovered that she runs the Queen's wardrobe well in advance, and to a planned schedule. It's not a case of getting up and each day and wondering what to wear! The Queen's schedule is planned in advance, with outfits planned, made or cleaned and refreshed if need be, hats chosen and jewellery selected. She clearly has a comprehensive catalogue of all the Queen's jewels, as she has said that she presents her with some suggestions that seem appropriate to the outfit/the event/the people attending, and the Queen then lets her know which options she wants to go with., or tells her her own preference Clearly, for security reasons she doesn't say where the jewels are kept or how it works, but I am imagining she makes a weekly trip to the vault, takes and signs out everything she needs, then takes them to a local safe where she can bring them to the Queen each day as she is getting ready for each engagement. She brings them to her in the same baskets that the Queen's grandmother, Queen Mary used.
For obvious reasons it is not the same as someone needing an emergency stapler. Also, I would have thought for obvious reasons, there is a clear process that would need to be followed to ensure that the location of each piece of jewellery, and who has assumed custody of it, are known at any point in time.
So, Meghan and Harry were at Buckingham Palace. It sounds like Angela Kelly was not. Who knows where the tiara was? If we're speculating, as you are doing, it could even have been at the Queen's personal jeweller in Tonbridge Wells to be cleaned before the wedding, as it hadn't been worn for decades.