The only piece I remember with a stipulation about not altering it is Queen Alexandra's necklace from her father. I expect it might be worn more if it could be adapted.
Interesting to note that the two Turkish gift diamonds are now set in the circlet of the QEQM crown and in the waterfall brooch.
I think you might clarify your descriptions, Boffer, if you referred to the 'circlet of Queen Mary's crown' and the 'circlet of Queen Elizabeth's crown.' We shouldn't give the impression to various readers, some of whom are not educated about royal jewels, that there are more circlets than there actually are.