I always thought the author meant it as "Etoile" which means star in french.. Either way, thank you so much for translating and feel free to do whatever with the name!
Etelle seems feminine and Ethel seems gender neutral. I’m fine either, though I feel that Ethel would be ironic since when she met Ernon she was pretending to be a boy.
Ethel is a traditionally feminine name so idk what all y'all are talking about with it being gender neutral. But +1 for Etelle (or maybe some derivative) because if Ernon can exist Etelle can too.