While I attempt to compile some worthwhile data for this family. I have a strong desire to tie Benjamin Lewis Furnell as the son of John Fernald and Miriam Lewis.
I saw mention of a Yarmouth birth record for Samuel Fernald b. 4 Apr 1802, son of Benjamin Lewis Furnel and Hannah Furnel. Church Records of West Yarmouth, MA Page 289 Published 1953. (It's hard for me to resist the possible Lewis connection!)
John Fernald married abt 1759 in Kittery to Miriam Lewis and had 4 children: 1. John b. abt 1761, 2. Miriam b. abt 1767, 3. James 4. Benjamin born bef 1773.
Miriam 2nd married Lt/Maj. William Haley on 22 Oct 1773.
John and Miriam's youngest son Benjamin was a minor (under 16) at the time his fathers will was probated, May 13, 1773 and supposedly when the estate was divided in 15 April 1782. If he was 14 in 1782 that would make him born about 1768 and about 19 in 1787. So he might just fit with Benjamin Lewis Fernald b. abt 1762 (date according to a worldconnect source on rootsweb.)who was married on 19 Nov 1787, in Yarmouth, MA to Jedidah "Hannah" Baker b. 20 Apr 1764. He fits better if he was only a minor when the will was 1st probated.
It's just a guess, but a promising clue to follow!