I still don't see what the big deal is.
The death of Lily's parents is unremarkable in the extreme. They were muggles.
As for James' parents, there's so much we don't know that it's really not worth trying to guess. For example, how does meopause work for witches? Why must we assume that they had to be under the age of 60?
The whole point was that it was not important. How many trivial sub-plots can one author follow?