I am not totally au fait with the faces of all the ladies in the extended royal family, but I think you are correct and that the lady is the Marchioness of Carisbrooke.
Below is another shot showing her face.
And I think you are correct and that she is wearing the Teck ears of wheat tiara.
By 1950 Princess Alice had reconfigured the tiara yet again and was wearing it in that formation.
For example in --
November 1950 (ed for a typo - I had 1952!)
And November 1952
I didn't spot Princess Alice in the gathering so it is possible that she had lent the tiara to the Marchioness, who did not have many jewels at her disposal.