This doesn't directly answer your question, but here are observations:
1) I happen to have an image of the 1784 tax lists for Amwell [from FHL film #865475]. It is organized into groups with surname beginning with the same letter. Topping of the 'J' group is:
Johnson, Benj., 180 acres improved land, 29 acres unimproved, 4 horses, 6 cattle, 5 white people in the household
Johnson, Jacob, 250 acres improved land, 44 acres unimproved, 4 horses, 6 cattle, 5 white people in the household
Johnson, Isaac, 147 acres improved land, 4 horses, 7 cattle, 4 white people in the household and 1 black person
Then Thomas Jones.
Then Johnson, Martin 68 acres improved land, 2 horses, 2 cattle, 6 white people in the household
Then 6 lines down: Johnson, Jacob [S B?], 180 acres improved land, 3 horses, 3 cattle, 9 white people in the household
[skip one line, then next page begins]
Johnson, Henry, 85 acres improved land, 4 horses, 4 cattle, 7 white people in the household
And that's the end of the J group.
2) The Benjamin Johnson whose profile you linked would seem a great candidate to be the one in the tax list. His profile incorrectly says that his will was proved in 1794 — the linked source indicates that he died intestate, without a will. But the administrators Isaac, Jacob, and Benajmin have the same names as the first three heads-of-household in the tax list.
3) This deed gives the heirs of Benjamin who died probably shortly before 1796, including the Isaac, Jacob, and Benjamin so named.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSK7-5S6F?i=47&cat=220656
Unfortunately, it doesn't seem a widow was a signatory. And I don't see a deed in the Early Land Records database on which Benjamin was grantor, which could give her name.
Conclusion: My feeling is that since you have a candidate Benjamin Johnson who was probably of the right generation to be the son-in-law, and living in the same township, and existing records are consistent with there being a single man of this name and no others, that it is okay to view this Benjamin as the son-in-law of Johannes Schwalluw with an "uncertain" flag... except that WikiTree doesn't have an "uncertain" flag for spouses, so it would just need to be written into the biography. His wife's forename could be "unknown," but I wouldn't be surprised if more digging turned up a candidate forename for her. Forenames are easy enough to change on Wikitree if one does turn up.