What plausible explanation can we find for the points of contact between Moses in the Pearl of Great Price and the Book of Enoch?

The books of Enoch, even The Secrets of Enoch which you quote here, are not as isolated and rarely consumed as the question suggests. In Scripture and Scholarship in Early Modern England, “Og King of Bashan, Enoch and the Books of Enoch: Extra-Canonical Texts and Interpretations of Genesis 6:1-4,” Ariel Hessayon writes:

Far from being neglected, Enoch and the books under his name had preoccupied monks, chroniclers, rabbis, Kabbalists, Academicians, magicians, Catholic theologians, Protestant divines, Orientalists, sectarian and poets alike. So much so, that by the mid-eighteenth century the available evidence in Greek and Latin had been exhausted.

Indeed, in his article Joseph Smith, Mormonism and Enochic Tradition, Salvatore Cirillo exhausts several important responses to your question. For a more complete answer that is far beyond the scope of this site, please read his article in its entirety. The article specifically addresses your points 1 & 4, and addresses the others in general.

Firstly, Cirillo dedicates a significant amount of effort to describe the trade of books between Europe and the Americas at the time, especially those of the Enochic narritive, in order to provide a firm foundation for the remainder of the discussion of the purported similarities between the texts.

Secondly, Cirillo addresses claims by proponents of Smith’s testimony on the texts, namely Hugh Nibley and D. Michael Quinn. Cirillo definitively responds to Nibley's conclusion that the only possibility for the parallels is divine revelation in the face of his claim that Smith lacked access to the Enochic literature. He says of Quinn (parenthetical notes are Cirillo’s):

Quinn instead suggests that ‘independent discovery’ (the idea that coincidental development of the same or similar idea can occur from separate persons or groups unaware of the other‘s work) is the best possible explanation for the argument for parallels. However, this is an equally difficult argument to posit when the parallel evidence is coupled with possible access.

Cirillo continues to go on that Quinn, despite his Mormon faith, lists five ways the Enochic literature was made accessible to Smith:

1. an advertis*m*nt for [Thomas Hartwell] Horne‘s book [An Introduction to the Critical Study and Knowledge of the Holy Scriptures] in a Palmyra paper;
2. another advertis*m*nt for Horne‘s book in a nearby Canandaigua bookstore;
3. yet another advertis*m*nt indicating Horne‘s book was continuously on sale in Canandaigua;
4. Horne‘s book told Palmyra‘s residents that the BE [Books of Enoch] was important; and
5. an American printing of Laurence‘s 1En [First Book of Enoch] in 1828.

Quinn concludes that, Nibley understated the access of Palmyra‘s residents in the mid 1820s to information about the pseudeupigraphic Enoch.

As another point in his thesis, Cirillo exhibits Smith’s dependence on the documents in question using the legal standard of substantial similarity. He concludes,

Substantial similarities and influence as argued by the Stanford study both support this thesis‘ larger argument that Joseph Smith‘s EPE [Extract of the Prophecy of Enoch] was influenced by the BE [Books of Enoch]. Hugh Nibley‘s assumption that Smith‘s EPE was the result of independent and wholly divine revelation is not viable. There is clear evidence which favours access by Smith to materials related to the BE and, as this chapter has shown, there exists a degree of substantial similarities not easily dismissed as coincidence. The use of the Son of Man motif, the relationship of Enoch and Noah, and the accounts of the journey‘s to question Enoch between Mahijah, Mahaway, Methuselah and Noah, all establish enough substantial similarity to argue that Smith was influenced. That each of these ideas and there expressions was only available in the BE accounts at the time of Smith‘s writing is proof positive that influence did occur.

Aditionally, Joseph Smith, before the final publication of the completed Book of Moses in 1851 had access to the content of all of these texts in addition to a faithful convert to assist him in the rabbinically trained polyglot Alexander Neibaur. In Joseph Smith and Kabbalah: The Occult Connection, Lance S. Owens writes:

By 1842 Joseph Smith most likely had touched the subject of Kabbalah in several ways and versions, even if such contacts remain beyond easy documentation. During Joseph's final years in Nauvoo, however, his connection with Kabbalah becomes more concrete. In the spring of 1841 there apparently arrived in Nauvoo an extraordinary library of Kabbalistic writings belonging to a European Jew and convert to Mormonism who evidently knew Kabbalah and its principal written works. This man, Alexander Neibaur, would soon become the prophet's friend and companion.

Later, he says:

Newel and Avery note in their biography of Emma Smith, "Through Alexander Neibaur, Joseph Smith had access to ancient Jewish rites called cabalism at the same time he claimed to be translating the papyri from the Egyptian mummies." That he not only knew something of Kabbalah, but apparently possessed a collection of original Jewish Kabbalistic works in Nauvoo, is however documented in material almost totally overlooked by Mormon historians.

And so an argument that the Enochic literature and other writings of Kabbalah and Hermeticism influenced Smith’s later writings is simpler to establish. As a whole, the manuscripts of Smith's dictations and writings used to compose the Book of Moses show "signs of subsequent correcting, editing and amending" and "are full of errors, omissions, and revisions." The Book of Moses was not published in its entirety until 1851, some seven years after his death. As far as the remaining two (4 & 5 Enoch) of the four other volumes you describe, perhaps you can edit your question to clarify exactly what texts you're talking about. Standard catalogues of biblical pseudepigrapha list only 1, 2, and 3 Enoch.