Do you think he’d lend it to William or Harry’s wives if they occasion suited?
To be honest, Kate has access to all the major tiaras as we go on. The other one wont ever have need of a tiara again.
She will eventually have access to those that belong to the monarch. Until that time, she only is able to wear what has been lent to her by the monarch, or anything that she may have been given (none, to my knowledge). The BRF doesn't have a jewel foundation, unfortunately. The few outright gifts have been for those who are not on the consort line (Sophie and Fergie), while she lends to all others at her discretion. Particularly to future consorts. The BRF tiara vaults are huge but we simply don't see other things; it seems pretty clear that the regular working royals have a specific set of tiaras to use and no more. And no swapping or borrowing or sharing like in Sweden.
Of course, we'll likely never know for sure how they handle jewels, so this is all speculation.
Diana did wear the Spencer Tiara for quite awhile until her brother basically had to demand it back, so journalists who fail to do due dileigence have assumed it belongs to the BRF.
How they handle the jewels, and the loaning of them changes with each new Monarch. Diana was loaned only the Cambridge Lover's knot, but Kate has both the Lotus and the CLK. There is speculation about the Cartier Halo Scroll tiara being lent to Kate but it went on tour shortly after the wedding and we've seen hide nor hair of it on anyone since then. It's never been a favorite of any of the BRF ladies so it's best left as a trainer tiara for Charlotte anyway.
When Charles becomes King it will be his turn to distribute the loot. Cams already has the big gun tiaras from the Greville bequest and she wears them splendidly - It will be interesting to see which of the other tiaras she cares to use. I think she'd look splendid in the Vlad.
It also remains to be seen if Charles and Cam step up tiara events in the future. If in times to come they do have the York princess' and the Wessex cousins in for state banquets and such they may very well have a loan of a tiara, and some other bling from the vault. Best way to do things, IMHO.
It's hard to tell when an item is a one-time loan (goes back to the vault), and when it's a so-called lifetime loan (presumably kept by the wearer). If I recall correctly, there were a lot of assumptions that Diana had been given the CLK since she used it exclusively, but that was obviously not the case. I suspect that the Halo was a one-time loan for Kate. It's lovely but was sent out on tour so quickly, and even then was really too small for a future consort. She wore the Lotus next, but even that was only twice, and not in 5 years. And then there's the significant pieces that the queen has lent to her - the wedding gift bracelet, the ruby and Nizam necklaces, and various earrings. Some of them were clearly for the event alone.
I think the only Greville tiara has worn is the large Boucheron, or "wastebasket" as some call it.
She's worn the Dehli Durbar once, and she's worn her own Cubitt tiara occasionally as well. She's a good tiara-wearer! I'm not sure she can carry off the smaller ones, though, since her hair already has such volume. But since her position will only get higher I expect to see her in other big guns. I'm sure she'll handle them well.
I really, really hope that C&C open up the vaults more. There's so much available, far beyond tiaras. I suspect that Camilla really likes the modern pieces, citing her extensive Van Arpels collection, presumably gifts from Charles. But there are so many diamond earrings and necklaces without super-sentimental or symbolic significance that she and Sophie and Kate could be wearing on a regular basis. If not for day events, definitely for evening events. She's got 3 or 4 nice diamond necklaces from the Saudis, plus who knows how many "plain" collet diamond necklaces. Spread them around!