The only thing I can say for certain is that your grandparents married in Pontypridd in the first quarter of 1918 - here's the record on Familysearch
https://www.familysearch.org/ark:/61903/1:1:26JF-76R
It is possible that it's only a coincidence that Percy and William James each had the surname Hunt. There are several families named Hunt in and around Pontypridd in the early 20th century. Perhaps they weren't brothers, but cousins of some kind, or maybe they were not related.
William James Hunt, born Pontypridd in 1899, was the son of George and Demara/Demaris Hunt. William died in September 1918, at least six months after your grandparents married - is it possible that htis individual isn't actually your grandmother's fiancee? Or that the family story has been amended over time?
If I found the right Percy Hunt (sorry, no guarantee!) he was born Pontypridd in 1897, and seems to have been the son of David and Emily Hunt (his later stepmother was named Sarah Elizabeth).
Both of the families lived in the Pwllgwaun district of Pontypridd.
This is giving you more questions than answers, isn't it? If I were you, I'd first check out all the evidence you have for the deceased fiancee of you grandmother, and make sure that you know for certain who he is. If you can't be certain, keep an open mind..
Next I would trace as much as possible about the family of Percy Hunt - if he did have a brother who was killed prior to Percy's marriage to your grandmother, perhaps that is your guy.
If you can't find a likely brother, then try to find any relationship between Percy Hunt's family and William James Hunt's family.
Good luck!
Dave