Roberts says that QEQM gave QEII the Halo tiara for her 18th birthday.
Personally, I cannot see why the Queen Mother would give her daughter a tiara if she would then not allow it to be worn until after marriage. It doesn't make sense. All that we know is that there is not a photo in public circulation showing QEII wearing a tiara prior to her wedding. There may be a number of reasons for this. Even after her wedding, when she had 3 tiaras at her disposal, QEII often didn't wear a tiara when others did.
Thank you for posting the photo of Princess Margaret Marianne. There are others from this occasion but I don't have a copy any more. I think that Princess Margaret wore the Halo tiara on at least 2 occasions in the Netherlands in 1948.