To be fair, Diana alternated the CLK with her own family tiara, so she had two to chose, like Kate currently, as I believe the Halo Scroll is still on tour? (And even if it was not, it's one of the smallest and ugliest HM has on offer, with those very visible sectioning it has in the base).
I would have loved to see Kate yesterday with Queen Mary's Sapphire bandeau, it would have been the perfect match to QEQM's earrings - and the last wearer was Princess Margaret, so it has been hidden in the vault for decades.
There are a lot of smaller (and some bigger, like the Teck crescent which is rumoured to be in Camilas hands) options available which have not been worn for 40+ years, and, as we have seen with Eug's Emerald sparkler, there are obviously even more no outsider has even an idea of.
So even if HM insists in solely wearing her "own" 10 or 12 most preferred items, she could make more loans if she would like to.
We will probably have to wait. I wonder what Camilla will choose to wear, and which jewels, including necklaces and earrings, she will lend out when the time comes.