So then why do / did you think that things would be different with it being passed on to Fred? That Fred would allow his wife to use it while Crown Princess and then Queen, but then when Christian marries, designate it for his wife. That would track if Queen Mary was deceased when Christian marries, or Mary never wore it or rarely wore the parure indicating that she did not care for it, but it's been her most-worn tiara.
My interpretation of your comment is that the ruby parure is intended for the Crown Princess, and if that's the case, then it would have been passed on to Crown Princess Margrethe when she came of age.