I too have been puzzled by how the tiara would look if the elements were removed.
I think the key to it lies in the comment in the Garrard ledger that "the outer frame ornament" could be removed. I take that to be line of diamonds at the base plus the undulating row of diamonds with the leaf elements.
That would then leave the tiara to be worn on a band much the same as the Girls of Great Britain was when given to QEII as a wedding present.
What some call the arches I think are called the spires and the bush ornaments refer to the upright elements. In their place large diamonds could be mounted to fill the gap.
If my interpretation is correct the tiara would then still have the central flower shapes with a large diamond at the end of the diamond rows which support the flower shapes.