Well, thanks to info posted here so far, I went onto Google maps and found a lot of the roads within I&SP have been mapped! (In 2018 only). I went to street view and actually found the headstone for Daniel's nephew Sidney Charles Harper and his wife Florence Rosa Harper by overlaying the cemetery map over the google map to pinpoint Z3C in High View Rd, west side

. Burials for each were in 1939 and 1974 respectively; I can
only just make out the names but not the whole inscriptions. 2nd photo shows facing west.
Still no luck for Daniel or Josiah, though... as it seems the gravestone plot sections are roughly in date order, I'm guessing they may be in the overgrown bush (that Chris mentioned) immediately south of, and behind the roadside graves in Z3a where Parkers and Raleigh Rds meet at the hairpin corner, if they still exist.