I’m quite fond of some of the Discworld variants of this, like Visit-The-Infidel-With-Explanatory-Pamphlets and Smite-The-Unbeliever-With-Cunning-Arguments.
At school in the 70s there was a boy called Nellie - short for Neill. Well, not short at all really, but used as a diminutive!
Maybe. The first English Eleanor was very likely Eleanor of Aquitaine. Who was named after her mother Aenor - and was called Alienor (another Aenor).
However, in English the similarilty of the names has created a relation between Ellen, Helen and Eleanor that really doesn’t exist from the derivation of the names.
I’ve used Eleanor as a form of Helen, but honestly, I have my doubts that the derivation of Aenor comes from Helen.
I suppose another long form could be “Nellybelle”, like Pat Brady’s jeep on the Roy Rogers show.
(my wife’s mother has trained our 2-year-old to say “Whoa, Nellybelle!” every time the car hits a dip going fast.)
On a tangent, how did Peg/Peggy become a nickname for Margaret? I can see Meg, but Peg?
Same way Bill became sort for William. Or Bob for Robert.
Margaret - Meg - Peg…William - Will - Bill…Robert-Rob-Bob.
This was very common in English culture –
Robert - Rob, Bob, Hob, Nob, Dob
William - Will, Bill
Roger - Rodge, Hodge
Richard - Rich, Rick, Dick
Nelly also used to be “short for” (ie: slang for) effeminate.
I call my daughter, Janelle, Nellie sometimes, although usually Nellie-Belly or Nellie-Welly (she’s 4). 
I have a Peace Smiley in mine.
And surely that’s got to be exactly how Margaret became Peggy. Margaret, Meg, Meggy, Meggy-Peggy, Peggy.
That’s how our dog Hoover became Doobie (Hoover, Hoover-Doover, Hoobie-Doobie, Doobie). 
You mean they didn’t just make that up on Arrested Development to be weird?!
I know a couple of guys who are named “Nelson”, they both go by Nellie (or Nelly)
Nope, apparently it dates to at least 1961.
Nellie is a common nickname for Eileen in my family as well. I too had an aunt Nellie who turned out to be Eileen - I only found out when she wrote me a check for something and we had the same name. Some of my older relatives will call me Nell or Nellie, but it seems to be fading out with the younger crowd.
Edward - Ed, Ted, Ned
Nellie, Nancy, Mary are all slangs for men acting like girls.
So I guess it is kind of just short for any name with an “n” sound that is emphasized.
There’s an old mill by the stream…Nellie Dean…
I know several Aussie Nellies who are actually Narelle.