This is an exceedingly complex sentence, connected originally by and’s. The resulting text is a long, incomplete, and complex subordinate after-clause (with parenthetical phrases and intervening relative clauses) that was twice extended by adding a connective and (listed above as 1 and 2). In his editing for the 1837 edition, Joseph Smith attempted to alleviate the difficulty by removing these two and ’s. In the printer’s manuscript, Joseph actually changed the second and to that, but the 1837 edition ended up simply deleting the and (or alternatively, omitting Joseph Smith’s that). Joseph’s that might not have made much sense between the two phrases headed by because of since both refer to the removal of many plain and precious things from the gospel. Nonetheless, the that could be interpreted as the beginning of the complement for the earlier “thou seest”:
thou seest …
that because of these things which are taken away out of the gospel of the Lamb an exceeding great many do stumble
The use of the first and to connect the subordinate after-clause and the following main clause (the one beginning with “thou seest”) may be viewed as a Hebraism. See the discussion regarding hebraisms in volume 3 as well as the examples listed under 1 Nephi 4:8–9 and 1 Nephi 17:50.
Examples like the second and are often found in the text when a long preceding subordinate clause is summarized by means of a shorter clause or phrase before giving the main clause, as in the following example:
The text in 1 Nephi 13:29 favors some connector such as and between the two phrases headed by because of, given that the second one summarizes the idea of the first.
Near the end of this complex sentence (listed as 3 above), scribe 2 of 𝓞 wrote & at the beginning of the main clause (that is, “and exceeding great many do stumble”). This use of “exceeding great many” seems incomplete without an indefinite article an. Elsewhere the text always has the indefinite article preceding “great many” (including two examples with exceeding):
(The indefinite article an in the Alma 13:12 example was mistakenly deleted by Oliver Cowdery.) Near the end of verse 29 in 1 Nephi 13, scribe 2 of 𝓞 apparently misheard the indefinite article an as and. His mistake in writing & for an provides evidence that the original manuscript was indeed dictated.
Summary: Despite its original complexity, the long sentence in 1 Nephi 13:29 with its two connecting and ’s will be restored in the critical text; such Hebraistic connectiveness is characteristic of the Book of Mormon text, especially when a subordinate clause is followed by the main clause; on the other hand, the last and, written as &, was probably the result of the scribe mishearing an as and.