Our younger daughter is Isla (eye-la). (Our older daughter is Helen, but that's out because of your H name veto.)
We know an Etta, which I think was a great choice for our close friends. Since they had their Etta, who is just Etta, we've met two others, both of whom used Etta as a nickname for something longer. One was Henrietta (H name, I know) and the other was Marietta which could sound cute with Calliope.
Cora and Clara were suggested upthread, and I really like both of those. Don't know if you want two C's though.
I like some of the other Greek suggestions to pair with Calliope.