I like
Nell much better than
Nellie, but I'm afraid that
Nellie would happen!
The only
Nell I've ever known was a Nel; her given name was
Cornelia, which explains the solitary L. But it's pronounced exactly like
Nell.
I've got
Nellie relatives on my father's and mother's sides:
Nellie was always the full name - not a nn for anything. This led me astray when I was in a genealogy phase!