The first pic for the State Opening of Parliament also shows the first two Cullinans worn as a brooch.

I quite agree that Queen Consorts do not change their garments during a coronation. But I am still wondering over whether Queen Alexandra actually wore her purple robe on the way to the Abbey. Your comment explaining the ermine wrap worn by Queen Alexandra in this pic encourages me to think the Queen might have added her purple robe on arrival at the Abbey. In this pic she is not followed immediately by any attendant juggling the long purple train. It is quite difficut to be certain here.