Hi Jersey Girl,
I don't know if this is a killer blow to the Book of Mormon, but it is an interesting historical account that I believe hints at the origin of the book.
I don't think Rigdon wrote the entire book himself, though I do think he plagiarized considerable amounts of Spalding's work; he appears to have borrowed heavily from Spalding for the "War Chapters." Broadhurst has shown through his word-print studies a heavy concentration of correlations between the "War Chapters" and the Spalding manuscript.
What is missing in the Spalding theory is an account of the Anthon incident and other stories from Joseph Smith Jr's life, the visions of his father as compared to Lehi, and the story of Limhi and Ammon leading to the translation of the plates found within the book. I see that inner translation event as a direct reflection of Martin Harris and Joseph Smith Jr's early efforts.
However, back to the original point -- in 1826, Alexander Campbell, founder of the Campbellite “Restoration Movement” and early LDS church leader Sidney Rigdon's mentor, began work on a translation of the Bible, which he called the “Living Oracles,” into English, using words that tended to support his doctrine.
For example, Campbell used the word "immerse" instead of a more generic "baptize," and also made frequent use of the word "messenger" in place of "angel" in his translation.
This altered the "old-fashioned" concept of angelic wings and cherubic faces found on European paintings and minimized the concept of an angel as a separate spiritual species of representative of God in the human world. Angels became "fellow laborers" or "fellow servants in the gospel." While the term is a biblical one, it also overlaps the jargon of the Campbellites of the late 1820s and the language of self-identification of "angels" appearing to Joseph Smith, Oliver Cowdery, and others in the early group of "believers."
A distinction developed between Campbellite angels and Mormon angels, in that the followers of Campbell believed that angelic visitations were a thing of the past, but early followers of Joseph Smith Jr believed they still ministered to men. This argument appears to be echoed within the pages of the Book of Mormon in Moroni 7:36 Or have angels ceased to appear unto the children of men?
During the period of 1929-30, Sidney Rigdon was in the process of separating himself religiously and ecclesiastically from Campbell. Parley P. Pratt, who was then one of Rigdon's followers, spoke of the group as "Rigdonites."
Parley P Pratt describes a “vision” he had, and I think Pratt is here describing a “Rigdonite angel,” in this vision of his:
http://www.sidneyrigdon.com/1880PrtA.htm (with thanks to Dale Broadhurst)
I've concluded since this "Angel of the Prairies" Pratt speaks of foreknew the future establishment of the Church that the "Angel of the Prairies" who outlined to Pratt his then contemplated and now executed religious plan, was none other than Sidney Rigdon himself, and that this fact accounts for Pratt's failure to give the name of his host or the date of his first meeting with Rigdon.
Comparing this story with the events described in the D&C which refer to the restoration of the Aaronic Priesthood by an angel has led me to believe that the restoration angel was also Rigdon, as D&C 35 tells us that Sidney Rigdon came forth "even as John."
D&C 27:7 And also John the son of Zacharias, which Zacharias he (Elias) visited and gave promise that he should have a son, and his name should be John, and he should be filled with the spirit of Elias;
8 Which John I have sent unto you, my servants, Joseph Smith, Jun., and Oliver Cowdery, to ordain you unto the first priesthood which you have received, that you might be called and ordained even as Aaron;
D&C 35:3 Behold, verily, verily, I say unto my servant Sidney, I have looked upon thee and thy works. I have heard thy prayers, and prepared thee for a greater work.
4 Thou art blessed, for thou shalt do great things. Behold thou wast sent forth, even as John, to prepare the way before me, and before Elijah which should come, and thou knewest it not.
Taking all of these facts together, it is difficult to ignore the role Sidney Rigdon had in the founding of the Church, both in the coming forth of the Book of Mormon and establishment and development of Authority.