Ellen, Helen, and Eleanor are often used interchangeably. For the most part, Ellen is the diminutive of Eleanor. Ellen is the common pronunciation of Helen. Nellie can also be the diminutive for Helen or Eleanor.
Jean and Jane are often used interchangeably. This gets complicated when you have a daughter of each name. Janet is often also referred to as Jane but can also be Jennie or Jessie.
Mary and Marion both often get referred to as May, Mamie or Maisie while Margaret usually goes by Peg or Peggy, but can also be referred to as Maggie or Meg.
Catherine, Kathleen and Kate are generally one and the same.
Isabel and Isabella are one and the same but may be also be known as Bella, Belle, Sibby or Tibbie.
For men, the diminutives are also readily used. John may be Ian, Iain, or Jock.
George may be Jordy, Geordy or Dod.
James may be Jamie, Jimmy or Hamish.
As the saying goes, "Keep calm and carry on!"