Fenella has much more in common with
Prunella than the other -ellas you mentioned. They're both odd, obscure and particularly English; one contains the name of an unpopular vegetable, and the other the name of an unpopular fruit. I really like both of them, but I can't see them appealing to the general public, even though
Fen and
Pru are both appealing nns. It just seems that -nella isn't liked much. No idea why, as it's by far my favourite of those types of endings.