Unless the John who married Sarah Harris is really Emblem or close variant and Eltham is the bad transcription. That would eliminate him entirely and solve my problem.
So John is born in 1729 in Buckland, marries Margaret Brown, has a few children in HW, moves back to Buckland, where they both eventually die, Margaret in 1777 (buried in HW) and John in 1796 (buried in Buckland).
John and Margaret had a daughter Mary in HW in 1755. Haven't found out what happened to her.
Did she marry John Eaton on 11 May 1778 in Buckland? I had the marriage down for her 1st cousin once removed also called Mary, but the dates fit better for a Mary born in 1755.