To add to your negative evidence, she is named only as Joan in the publications listed for Matthew Whipple in Hollick's New Englanders in the 1600s:
As she is also the mother of John Whipple, I checked the publications about him listed in Hollick, with only one mention of her found:
- History and genealogy of "Elder" John Whipple of Ipswich, Massachusetts : his English ancestors and American descendants https://archive.org/details/historygenealogyo00whip/page/n12/mode/1up
Calls her "Joan (maybe Stephens)" but also says her parentage is unknown and mentions no evidence for the Stephens surname.
However the GMD entries for the brothers point to a very useful article:
- William Wyman Fiske, "The Whipple Family of Bishops Stortford, Hertfordshire: Proposed Ancestral Origin of Matthew Whipple of Bocking, Essex, and a Whipple Ancestral Line for Arthur Gary of Roxbury, Massachusetts," The Genealogist, Vol. 20, part 2, pages 191-217. https://www.americanancestors.org/databases/genealogist-the/image?pageName=191&volumeId=62791
This gives explicit evidence that the 1576 marriage record is fabricated: "The first Whipple entry in the Bocking parish registers (which start in 1558) is that of the burial of Margareta Whipple on 13 June 1577, followed six years later by the baptism of Matthew's firstborn, Anne, on 1 September 1583" (p.192)
Fiske also says "Although an extensive independent search of Essex, Hertfordshire and London records was conducted by the author, virtually all of the data … regarding Matthew's parentage, can be found in a single source: Holman and Marvin" (p.192 n.3). Holman and Marvin refers to Abstracts of English records gathered principally in Devonshire and Essex in a search for the ancestry of Roger Dearing, c.1624-1676, and Matthew Wipple, c.1560-1618
This, however, is the most informative statement: "… Hercules Stephens may have been a brother-in-law, as all others mentioned by Matthew [the father of the immigrant brothers] in his will were direct relations … The unusual name Hercules was chosen by Matthew's daughter Margaret, and her husband Laurence Arthur, as the name of their eldest son … While it is possible that the Joan Stephens baptized 15 Nov. 1562 in Bocking … was in fact the wife of Matthew Whipple … and a sister of Hercules, there is no direct proof at present." (p.191 n.2)
I'd say the case for LNAB as Unknown reflects currently known evidence, but the proposal for Stephens needs to be on her profile.