I have found another source for the marriage between Darby and Agnes, but again no surname for Agnes is given. This source also supports the remarriage of Agnes to William Williams:
"It appears that William Williams had married the widow of Darby Field, whose name was Agnes, as a deed shows."
Later in this volume it states:
"A William Williams married, in Taunton, Eng., 27 Aug. 1615,
Thamzyn Hill, and a man of the same name married at the same
place, 4 Feb. 1628, Agnes Newton. This may be the William
Williams who came with Thomas Wiggin to Dover Neck in 1633.
He married, second or third, the widow of Darby Field, and her
name seems to have been Agnes, for William Williams and wife,
t Agnes, signed a deed 18 June 1674."
HISTORY of the TOWN OF DURHAM NEW HAMPSHIRE (Oyster River Plantation) WITH GENEALOGICAL NOTES, By EVERETT S. STACKPOLE and WINTHROP S. MESERVE, Volume Two, pages 191 389.