I don't know anything about the tiara MarcA.
The necklace is definitely a Cartier piece and I think it is now part of their collection, although it was not shown in Canberra.
According to Hans Nadelhoffer, Cartier it was made by Cartier Paris and commissioned in 1909. Nadelhoffer has a photo showing that the central pendant is detachable to wear as a brooch-pendant.
At the Canberra Cartier exhibition there was a series of plaster casts displayed in one of the rooms. I am fairly confident that one of these was of the Vanderbilt necklace. I took these two photos.
The large rose brooch Mrs Vanderbilt was wearing on the centre of her corsage is the one which belonged to Princess Mathilde Bonaparte.
Vincent Meylan's research shows that the brooch was retailed by Mellerio, Paris. After the Princess's death her jewels were auctioned, the brooch purchased by Cartier and, according to Nadelhoffer, sold to Mrs Vanderbilt in 1904. It is now part of the Al Thani collection.
Vincent Meylan has a superb photo of it on his Instagram site
https://www.instagram.com/p/Bn2EbBrBRy4/?taken-by=vincentmeylan