I love some of your ideas, especially going "full on nun." My husband had a memorable teacher, Sister
Finbarr. I love the idea of
Mary Finbarr in her honor.
As for your portmanteau idea, what about
Mary June, nn Moon?
Mary Adelaide, nn Milady?
Mary Beatrix, nn Maybe? OK, so no one would really call a child Maybe. But it's a fun hypothetical exercise.