Thanks, Chris. That is a very lucid and succinct argument that strikes me as being very strong.
There are basically three accounts that can reasonably be construed as supporting a long scroll:
1) A story attributed to Joseph F. Smith says that one of the papyrus scrolls, “when unrolled on the floor, extended through two rooms of the Mansion House.” But this quote is known only from a casual comment by Hugh Nibley, who heard it from Preston Nibley, who heard it from President Smith, who was recalling a time when he was five years old or younger. And in addition to its rather incredible provenance, it differs substantially from Hugh Nibley’s own earlier telling of the story, which had President Smith remember “Uncle Joseph” seated on the floor of the Nauvoo House (not the Mansion House) with “Egyptian manuscripts spread out all around him.” Like the “one that got away”, this tale seems to have taken on new proportions in Hugh Nibley’s memory.
2) Charlotte Haven’s account is stronger, but still not definitive evidence. Haven wrote to her mother about being shown the mummies and papyrus by Lucy Mack Smith in March, 1843. Haven related that Lucy “opened a long roll of manuscript” that she identified as “the writing of Abraham and Isaac.” Since Klaus Baer’s estimate of 60 cm for the interior portion of the Hôr scroll is hardly “long” by Egyptological standards, Haven’s report seems to imply the presence of another text following the Document of Breathing on the scroll. What we must keep in mind, however, is that Charlotte was completely ignorant of Egyptological standards. She had no referent against which to judge what constituted a “long roll”. Indeed, her use of the term “manuscript” to describe the papyrus may indicate that she evaluated the scroll’s length relative to typical nineteenth-century paper manuscripts rather than to typical Ptolemaic papyrus scrolls.
3) In my paper I provide a third piece of evidence, which as far as I know has not been cited by apologists. An 1857 summary of the contents of Wyman’s Museum mentions “papyrus scrolls,” then clarifies, “some large fragments of Egyptian papyrus scrolls, with pieratic (priestly) inscriptions, and drawings representing the judgment of the dead, many Egyptian gods and sacred animals, with certain chapters from the old Egyptian sacred books.” This reference, however, is vague, and it's unclear how large or fragmented the "scrolls" described are supposed to be.
This evidence would be enough to persuade me of the length of the missing papyrus if not for the contrary data. Several lines of evidence appear to contradict the hypothesis of a large missing Hor scroll. Briefly, they are as follows:
1) An early Egyptologist named Gustavus Seyffarth viewed the missing papyrus in 1856 and described only the Hor text and Facsimile 3. He gave no indication of another text on the scroll, and in fact explicitly denied that the scroll contained a record of Abraham.
2) Klaus Baer predicted that the missing portion of the Hor text would be around sixty centimeters. When Andrew Cook and I did our analysis of scroll geometry, our estimate of the missing length agreed almost exactly with Baer's estimate. We were actually rather shocked at how closely the two figures agreed. In fact, based on the Haven accounts and the museum catalog, I had actually expected to find the missing portion was longer than that.
I know some readers have felt I'm being too dismissive of the accounts that are favorable to a longer missing scroll, but I'm being only as dismissive as I feel the better part of the evidence requires me to be. The close agreement of Baer's estimate with our geometric analysis, coupled with Seyffarth's scholarly report on the scroll's contents, is far more specific and concrete than the vague nineteenth century references to a "long" roll or "large" fragments.
Anyway, the length of the missing portion of the scroll is pretty irrelevant from an apologetic standpoint, since there are also several lines of evidence agreeing that the extant fragments were the ones used to translate the Book of Abraham. I briefly describe that evidence in my JWHA paper.