I've seen photos of Pss Michael in only two tiaras, one of which seems to have two versions.
As a refresher here are pix and sources.www.thecourtjeweller.com/2017/12/the-daily-diadem-argyll-diamond-daisies.htmlwww.thecourtjeweller.com/p/kent.htmlwww.princessmichael.org.uk/photo-gallery/www.thecourtjeweller.com/2018/05/the-daily-diadem-lady-fredericks.html
The Argyll Diamond Daisies: A group of diamond brooches in daisy and leaf shapes, sometimes grouped together for wear as a tiara-like ornament. Originally belonged to Princess Louise, Duchess of Argyll (who wore them as brooches at her wedding in 1871). Later bequeathed by her to Princess Marina, Duchess of Kent, and then passed to Prince and Princess Michael of Kent. Worn as a tiara-like ornament by Princess Michael of Kent.
The Kent City of London Fringe Tiara: A classic Russian-inspired diamond tiara made of a series of fringes and spikes. Given to Princess Marina, Duchess of Kent as a wedding present from the City of London in 1934. Inherited by Prince Michael of Kent in 1968. Worn by Princess Marina, Duchess of Kent; Princess Alexandra, the Hon. Lady Ogilvy; and Princess Michael of Kent. Worn as a wedding tiara by Princess Alexandra (1963) and Princess Michael (1978). [Note: there are two versions]
The Kent Pearl Festoon Tiara: An elaborate, elegant tiara of diamond scrolls and festoons, embellished by a series of round pearl toppers and a row of round pearls at its base. Originally belonged to Princess Marina, Duchess of Kent. Inherited by Prince Michael of Kent in 1968. Worn by Princess Marina, Duchess of Kent; Princess Alexandra, the Hon. Lady Ogilvy; and Princess Michael of Kent.
Sophie Winkleman wears a tiara and (a matching comb attachment for her veil) at her wedding to Lord Frederick Windsor, September 2009; I don't think we've ever heard any additional information about either jewel.
Maybe she'll take some existing jewelry and have a new one created especially for Ella to wear at the wedding?
Then Pss Michael can wear it later.