I'd like to propose that Mercy Hall was the daughter Marcy of Isaac Hall."A baker by trade, came over from England about 1722 and settled in State (then King) street, Boston, but removed to New Meadows or Harpswell, about 1737." History of Thomaston, Rockland and South Thomaston
Isaac Hall had a daughter Marcy born 1-27-1722/23 in Boston. Boston, New North Church records 1714-99
He also had sons Nathaniel b. June 11, 1727 and Isaac b. 1-31-1724 in Boston Boston, New North Church records 1714-99
and son Joseph Ancestry.com World Tree
In the 1790 census you will find Ebenezer Toothaker living quite close to Nathaniel,Joseph and John Hall.Also next door to Ebenezer is a William Hersey.One of Ebenezer's descendants married a William Hersey, and Isaac Hall (III) , b. 1757 in Harpswell m. Elizabeth Hersey in Harpswell.
Also from the: Commisioned Officers of Col. Samuel Waldo's Regiment, 1762-7 Taken from the Massachusetts Archive, Vol 99, p. 52.
1st Co., in Harpswell, Isaac Hall, Captain Daniel Wood, Lieut. Ebenezer Toothaker, Ensign.
I think that the combination of close physical proximity in Harpswell of 4 of Isaac's children Marcy (Hall) Toothaker, Nathaniel, Joseph and John Hall, as well as the proximity and marriages of both families to the Hersey family, along with the service record above make a good case for Ebenezer's wife Mary-Mercy being Marcy, the daughter of Isaac Hall.
Note also that Ebenezer and Marcy had sons named Nathaniel and Isaac.