In asking Gordon B. Mitchell for some references regarding the "third hypothesis" discussed here, he kindly provided the following which I have rearranged and edited a bit.
From George D. Smith, An Intimate Chronicle; The Journals of William Clayton, p.513-515 Appendices A.
Extracts from William Clayton's [Private] Book--Keys
Authors Note: Joseph Smith began presenting the following "keys of the mysteries of the kingdom" to his twelve apostles prior to their August 8, 1839, mission to England. William Clayton, who remained in England another year, subsequently copied these discourses along with others he evidently recorded from the prophet in 1840-41. In 1880 L. John Nuttall, secretary to the First Presidency of the LDS church, made extracts from Clayton's account. Nuttall wrote on the inside front cover, "L. John Nuttall his Book 1880," and entitled the work as above. ...
By Joseph, January 5th, 1841
Answer to the question, was the Priesthood of Melchizedeck taken away when Moses died.Notes 7 These ideas of uncreated matter reflect contemporary science, although the sun is now recognized to have a beginning and a projected end. See also Journal 2, "Nauvoo, Illinois," n27.
All priesthood is Melchizedeck; but there are different portions or degrees of it. That portion which brought Moses to speak with God face to face was taken away; but that which brought the ministry of angels remained. All the Prophets had the Melchizedeck Priesthood and was ordained by God himself.
The world and earth are not synonymous terms. The world is the human family. This earth was organized or formed out of other planets which were broke up and remodelled and made into the one on which we live. The elements are eternal. That which has a beginning will surely have an end. Take a ring, it is without a beginning or end; cut it for a beginning place, and at the same time you have an ending place.
A key, every principle proceeding from God is eternal, and any principle which is not eternal is of the Devil. The sun has no beginning or end; the rays which proceed from himself have no bounds, consequently are eternal. So it is with God. If the soul of man had a beginning it will surely have an end. In the translation, "without form and void" it should read "empty and desolate." The word "created" should be formed or organized.7
The relevant excerpts about the "earth being formed out of other planets" are also quoted in:
Richards & Little, Compendium of the Doctrines of the Gospel, Ch.62, p.287
Frederick J. Pack, "Natural and Supernatural," Improvement Era 11 (January 1908), 182.
Other books of interest on Mormon cosmological speculation are:
Joseph Fielding Smith's Man, His Origin and Destiny
BH Roberts's The Truth, The Way, The Life: An Elementary Treatise on Theology
Erich Robert Paul's Science, Religion, and Mormon Cosmology
The alien-Adam concept was a belief that Roberts and Smith shared with the original source being Brigham Young.
Also see Evolution and Mormonism : A Quest for Understanding for more on the above subject written by active and believing LDS scientists.