Use French when no translation
As far as I understand, there are two families of Norman: in the Channel Islands, whose the Jèrriais, and in Normandy. In the Channel Islands, the English is more widely speak than the French, but in Normandy the French is more widely speak than the English. Also, the Norman is a langue d’oïl like the French, so indeed French is perhaps a better fallback than English.
Sorry for my small responsiveness. I see there are reactions on the 2 other languages. For Nourmande, I see there are no active users (only maintenance) and only a few editions. I remarked an anonymous Nourmande contributor who contributed yesterday, I left him/her a message on his/her talk page in the hope s/he will answer there or here. If there is no answer in the next month (or before if you want to do the merge), I’m personally in favor of the merge – and if someone in the future has arguments for other fallbacks, it can always be changed.