The Lovers Knot Tiara has 19 pearls on the band (+ possibly 19 extra upright pearls mounted on the top). Roberts mentions that the 19 upright pearls were made removable in 1932 (so two years before the pictures posted with Prince Paul and Princess Olga of Greece), but I understand that Garrard made a removable gold frame to support the upright pearls, which implies it is still possible to wear the Lovers Knot Tiara in its original version.
The Vladimir Tiara has "only" 15 pearls.
From the following picture (initially posted by Kongetro), my feeling is that we are probably closer to 19 than to 15 pearls. So I would bet for the Lovers Knot Tiara.
As for Mary, Princess Royal, Countess of Harewood, she is definitely wearing Queen Victoria's sapphires, presented to her as a wedding gift by her father King George V: