I find it rather strange that the Queen Mother would have organised for some changes yet is not thought to have worn the jewel. I saw in Roberts' footnote that there is a Garrard ledger entry dated 22 May 1937 for the work, but whether this is a billing date or the date the order was placed is not clear.
I have read, at one time or another, the various descriptions of HM's jewels for the Courts, Gala concert, State Ball and other balls around the time of the coronation and only one description made me wonder if the Queen wore the diamond diadem. It was at a ball given by the Duke and Duchess of Sutherland.
Source: Sunday Times 23 May 1937.
It is possible that she was wearing her own lotus flower tiara but, to my way of thinking, that is not a 'big tiara...', and would have been familiar to journalists.
It is also possible that the Queen wore the diamond diadem on occasions for which the Court Circular did not provide any description of jewels.
After the coronation, QEQM regularly wore her own crown for important occasions - such as the State Opening of Parliament in 1937.
ILN 30 October 1937
and the State visit to France
For the first Court she wore the Oriental circle tiara and rubies as she regularly did for other significant events such as State visits; the Opening of Parliament in Ottawa etc
Ottawa 1939
The Royal Collection site makes it clear that the diamond diadem was in QEQM's possession at one time but sadly does not provide any proof that she actually wore it. https://www.royalcollection.org.uk/collection/themes/trails/diamonds-in-the-royal-collection/the-diamond-diadem