Could you explain why «coquille» is not accepted for "shell"? In LaRousse, «coquillage» appears to primarily mean the group of animals, and can refer to only the soft living part inside the shell. Indeed the first definition of «coquillage» is «Mollusque revêtu d'une coquille».
https://www.larousse.fr/dictionnaires/francais/coquillage/19198