The tiara looks pretty wide on Queen Elizabeth, maybe too wide for her. That could be the reason it hasn't been worn much.
This is the reverse of what I said about the Girls tiara in another thread. A tiara looped into a circle becomes a coronet, and vice versa. But the Crescents coronet does look larger than the Girls coronet.
Does Roberts say how many roses were used in the coronet? If it was four, one may have been removed for the tiara. Is there perhaps such a brooch?