The last time that Queen Silvia has worn this tiara was in 2010 at the 70th Birthday of Queen Margrethe. And the blog has a pic of the Queen wearing it as a necklace in 2012, also in Denmark. I don't remember seeing the necklace on the Queen after that. So I reckon that you are right and Madeleine was the sole wearer of this jewel since her wedding.
Usually the court website calls it a 'private tiara', but there have been one or two occasions when they wrote the Madeleine wears 'her own tiara', and those were before she got married. Admittedly the court site sometimes make mistakes. Eg. they at first said that Madeleine wears 'her own tiara' at Victoria's wedding, when Madeleine wore the Connaught which definitely was not her own.
Personally, I believe that this is Madeleine's own tiara, and that she has owned it long before her wedding. Why shouldn't she share it with the mother, when she doesn't need it herself? But as a matter of fact, we don't have any official source saying if it's her personal property or not.