This is not my family & my sources are sketchy. However, Paul Starbuck b. abt. February 29, 1747/48 & Mary Coffin b. August 17, 1750 d/o Jonathan Coffin & Eunice Barnard were "reportedly" parents of Shubael Starbuck who m. Ibby Close.
I say reportedly because Shubael Starbuck is not mentioned in Hinshaw's Ency. of American Quaker Gen. on p. 839, Vol. 1 in the Deep River MM records when we find "1786,6,5 Paul Starbuck & w & eight ch (s: Seth, Paul, John, Matthew, William & Trustram (sic)) rocf New Garden MM, dated 1786,5,27 to Deep River MM, Guilford Co., NC 1786,6,5, Mary Starbuck & dt Eunice & Hepzabah rocf New Garden MM, dated 1786,5,27.".
So presumably Shubael was born after June 1786, but I don't see him in the birth records at Deep River MM.
Then on Dec. 2, 1811, at Deep River MM, we see that a Shubal Starbuck was disowned. My guess is that he was born after 1786 & that more work needs to be done to prove his parentage.
I have no source worth quoting for this, but there were reportedly 3 children born to Shubael Starbuck, James, Salathiel, & Martha, which would be consistent with the will record you mentioned.
Also, on October 27, 1816 at New Garden MM, Guilford Co., NC, a Paul Starbuck, presumably the same as the above & former h/o Mary Coffin, married a Hepzibah Russell. No idea if this was Hepzibah's maiden name or if she was the Hepzibah Russell b. February 14, 1776 d/o Timothy & Sarah (Swain) Russell; my hunch is that it was prob. a 2nd marriage for her too. Do you have a death date for Paul Starbuck or Mary Coffin by any chance?