I have just come across your message.
I have been researching the Elliff family for about thirty years. The John William Eliffe who married Martha Ann Gregory is my 5th cousin, twice removed. If he is your great grandfather, then I calculate that you and I are 5th cousins, once removed. I am attaching John's details from my database. You can find more about his place in the Elliff family on my website (address below).
Regarding the Harry Eliffe who died in WW1. At present I can't connect him to my Elliff tree. I am attaching a tree that shows what I know of his family.
I hope this is of interest. I would love to find out more about your immediate family.