I have an entire chapter on the Bernhisel letter in the second edition of the Zarahemla book that looks at this issue in depth. In my view, the letter is a perfunctory thank-you note that reflects Wilford Woodruff's thinking, not Joseph's. Maybe if we ever figure out who wrote the note, we'll have a better idea.
Nevertheless, Roper/Fields continue to claim Joseph Smith wrote or dictated the note. They've even included it in their corpus of Joseph Smith's writings for purposes of analysis.
In this post, I'm going to focus on just two paragraphs from the Roper/Fields/Bassist article. The Roper/Fields/Bassist are in black below, with my comments in red.
Now for the third part of Roper’s trifecta.
He assumes his theory is fact (i.e., that Taylor, Woodruff and others “used” the Stephens book, when the only evidence is that Woodruff read them and an anonymous person used them in unsigned editorials). Then he uses his unsupported theory to form a straw-man argument by turning my argument inside out. Nowhere have I written or suggested that Joseph Smith disapproved of these books. In my view, there is no evidence he cared about them at all, any more than he cared about Catholic Piety.
In addition, it is not inconsistent with what Joseph actually said and wrote to consider post-Book of Mormon ruins in Central America as evidence of post-Book of Mormon people who may have included migrants from Lamanite territory. In fact, the Aztalan article in the Times and Seasons makes that argument. Thus, there is no problem with people citing Stephens as general evidence of the Book of Mormon; the problem is claiming the text says Lehi landed there and that Zarahemla is in Quirigua (or nearby), which is the unique feature of the anonymous editorials published on Sept. 15 and Oct. 1.