Arminel actually has an entry in E. G. Withycombe's "The Oxford Dictionary of English
Christian Names" (3rd edition, 1977). This is:
A rare name which seems to be confined to
Devon, where it is still in use. There was an Arminel Cording in that county in 1797. It looks like a diminutive of
Armine, the English feminine form of French "
Armand",
German and
Dutch "
Herman."
Since the above entry just says Arminel "looks like" a diminutive of
Armine, I suppose it shows Withycombe was just trying to give an educated guess and really didn't have hard evidence for that explanation. But it seems to be a good possibility that her guess is correct.