From what Roberts says, I think the Teck crescents tiara can be dismantled. Probably it was cleaned before Queen Elizabeth took it to Canada, and put back together the opposite way. The added diamond rivieres had also been removed from the base, and the tiara no longer looked like a coronet. While it was at the jewelers, it must have been stretched out onto a wider base.
Yes, this book is about diamond jewels now in the Queen's collection. I'm just wishing for one that traces tiaras and major jewels like the Teck hoops necklace through the families.
The pendant/brooch worn by Princess Mary Adelaide in your third picture has a lovely history. The Queen wears it now and then.
Edit: We were typing at the same time. Yes, I think that is the Duchess of Cambridge brooch.