For the record, the full citation for the Richard Stillwell will abstract is:
Abstracts of Wills Vol X 1780-1782, p. 58; The New York Historical Society, 1901.
(From The New York Historical Society Collections, this 1901 volume features abstracts of wills on file in the Surrogates Office, City of New York, from 1780-1782).
It is an abstract, not the original. Folks familiar with these volumes are aware that they are rife with spelling errors, particularly with the spellings of names, but frequently with total omissions as well.
It makes no sense to make any decisions about a person's ancestry based upon how someone else may have spelled that person's last name in their will, or worse yet a will abstract. Proper genealogy requires something a little more substantial.