Are tiaras ever a need this day and age? Yet aristocratic families hold on to them and displays them out in the few occasions. I’m not sure it’s hard to fathom Harry’s line would have its own tiara to pass down. Especially considering that during his wife’s lifetime, they’ll get their money’s worth in uses. We know that significant pieces from the vault aren’t loaned out so easily these days. Sophie only has the five aquamarine one after years of no loans from the vault.
Meghan will have the same need Kate has, the big dinner in December and the odd state visit. Maybe twice a year. Unless she and Harry take over for Ed and Sophie doing weddings but there wont be any for years to come. There is no reason she needs more than one tiara. No reason that one of the tiaras not in use from the royal vaults wont be given to her.
The aristocracy is a different matter. Those tiaras were bought generations ago when tiara events were common. Heck even in Diana's day, Diana wore a tiara to the ballet. Many of these families hold on to them as tradition, and many have sold them (why we see so many tiaras in auction).
When Sophie and Edward married Sophie was never meant to be a working royal. The couple had continued their jobs and planned to be private. It may be one of the reasons she was given the tiara she was. When they became full time working royals, a second tiara was purchased from her (her necklace tiara) and eventually she also got the five aquamarines.
Meghan will be a working royal from the start. And when her father in law is king (likely in less then 10 years sadly), her and her husband will be the 'spare couple' until George takes on duties which will likely be another 20 years or so after his grandfather is king. Not same place as Sophie.
It's not entirely beyond consideration that one or two extremely wealthy Americans who have now made the UK their home might 'gift' a smallish, relatively inexpensive tiara to Meghan as a symbol of the so-called 'special relationship' between the two countries and the two individuals tying the knot. Unlikely I'm sure but not impossible.
Tiaras can be purchased relatively cheaply at auctions - look at that one Mary of Denmark bought for herself. It's underwhelming sure, but it does a job and it only cost around 10,000 Euros or something?
Meghan could wear a nice new tiara and all the Palace has to say is that it was a wedding gift - they don't need to elaborate on who bought it for her. We have no idea who bought Kate most of her jewels.
Personally I think it's most likely she won't wear a tiara at all - maybe some sort of 'boho chic' head ornament (like this maybe) or flowers.
Again it would smack of buying 'undue influence'.
If they were not clear on who bought the gift, it would cause more issues. There would be constant suspicion it was bought by tax money. Or if rumors came out, that it was indeed bought by Americans, that it was a bribe from Trump or something ridiculous. Not being straightforward would cause more issues then anything else.
They would be smarter to simply say Charles bought it as a gift for his future bride.