There are a lot of reasons that the missing scroll theory is DOA. We know where on the Hor scroll Joseph Smith thought the Book of Abraham was to be found. We even know exactly which Egyptian characters he thought contained the beginnings of the Book of Abraham. The Book of Abraham itself references the scroll on which it was thought to exist, which is "The Hor Scroll" or "Book of Breathings", and also known as P. J.S. 1 We have the beginning and the end of that same scroll. All characters on the KEP can be matched to extant papyri. All extant loose patches can be matched to extant pieces. Cook and Smith have given a "mathematical refutation that the scroll could have not contained a further text" and was only originally about 5'-0" long See here.
All known contemporary references to what was on the scrolls can be found on extant pieces. The two primary accounts used to describe long scrolls are problematic. Don Bradley has pointed out that Haven's account shows she always exaggerated , and the Nibley account relies on a decades old second hand memory of a five year old witness which cannot be verified. There is no evidence that even if the missing 22"± in the middle of the Hor scroll suddenly turned up that it would contain anything other than more funerary text. Estimates by Egyptologists call for about 15'-0" of scroll in order to contain the text in the Book of Abraham, a length not found in either of the two scrolls Joseph Smith obtained from Chandler. And so on.
Yesterday, while I was looking for the actual Egyptian translations of the Ta-Sherit-Min scroll, the one Joseph Smith identified as the Book of Joseph,
I came across this passage in The Joseph Smith Egyptian Papyri by Robert Ritner.
Robert Ritner in The Joseph Smith Egyptian Papyri wrote:Despite its association with the Book of Abraham, the content of P. J.S. 1 concerns only the afterlife of the deceased Egyptian priest Hor. "Books of Breathings" are late funerary compositions derived from the traditional "Book of the Dead." Like their antecedents, the sole purpose of the later texts is to ensure the blessed afterlife of the deceased individual, who is elevated to divine status by judgement at the court of Osiris and is thereby guaranteed powers of rejuvenation....
The modern designation "Book of Breathings" includes a variety of late funerary compositions, but the text found in the Joseph Smith collection represents a specific type termed in antiquity "The Document of Breathings Made by Isis for Her Brother Osiris". P. J.S. 1 is thus a formal document or "permit" created by Isis and copied by Thoth to assure that the deified Hor regains the ability to breath and function after death, with full mobility, access to offerings and all other privileges of the immortal gods. This specific form of "permit" was used by (often interrelated) priestly families in Thebes and its vicinity from the middle Ptolemaic to early Roman eras, and the limited distribution probably accounts for their uniform pattern, which displays only minor modifications.
As a result of this uniformity, the original size of the papyrus is not in doubt. With textual restorations and the now lost Facsimile 3, the papyrus will have measured about 150-155 cm.
Ritner, Robert K. The Joseph Smith Egyptian Papyri SLC The Smith-Pettit Foundation 2011 pp.86-87
In other words the Hor scroll in the Joseph Smith Egyptian Papers is a a copy of a specific document whose content and length is known from other copies of the same document.