It is possible here that the reading of the original manuscript is in error. Elsewhere in the Book of Mormon, as well as in the King James Bible, when the word God occurs in conjuncts involving Abraham, Isaac, and Jacob, there is a consistent aspect to the repetition of elements: either the full phrasing of each element is repeated, or once a word or phrase is dropped from the repetition, it is not used again. We have the following examples from the Book of Mormon and from the King James Bible:
David Calabro notes (personal communication) that all the King James examples of and before Isaac are found in New Testament examples, while that and is missing in all the Old Testament ones. He further notes that this systematic difference in the use of and in the King James Bible is found in the original languages (in the Hebrew of the Old Testament and the Greek of the New Testament).
The pattern of repetition is also found in other conjuncts in the Book of Mormon involving Abraham, Isaac, and Jacob:
There are also 17 examples in the King James Bible that support this same pattern of repetition. The example from Mosiah 23:23 provides support for the accidental loss of repetitive elements in the Isaac conjunct. The 1837 edition accidentally lost the preposition of, giving “the God of Abraham and Isaac and of Jacob”:
All these examples, including the 1837 loss of of in Mosiah 23:23, imply that the original text in 1 Nephi 19:10 may have read “the God of Abraham and the God of Isaac and the God of Jacob” and that the first repetition of God was accidentally dropped when Oliver Cowdery took down Joseph Smith’s dictation. In fact, for this conjunct the original manuscript (lines 25–26 of page 39) actually reads line for line as follows:
... yea the God of Abraham & of Isaac & the God ofJacob ...
After he had written “& of Isaac &”, Oliver started to write “the God of Isaac”. He crossed out that part of Isaac which he had already written (that is, Isa), then wrote Jacob inline immediately after. This corrected error suggests that Joseph actually dictated “the God of Abraham and the God of Isaac and the God of Jacob” and that Oliver accidentally dropped the first repetition of the God. The error would have occurred at the end of the line, a common place where errors in writing down dictation can occur.
There is one other case in the Book of Mormon that violates the normal pattern of repetition:
One could argue here that the repetitive element (namely, the preposition with) has been accidentally dropped, just like the preposition of in Mosiah 23:23. Unfortunately, the original manuscript is not extant for Helaman 3:30.
It may be that 1 Nephi 19:10 and Helaman 3:30 originally had the repetitive element before Isaac, but ultimately the evidence for emending the text in these two cases is not especially strong. (The scribal deletion of Isa in 𝓞 makes the argument somewhat stronger for 1 Nephi 19:10.) In both cases, the text is understandable even with the repetitive elements missing. Thus the most conservative decision would be to rely in each case on the earliest textual sources to determine the reading.
Summary: Based on the earliest textual sources, maintain the inconsistently repetitive conjuncts in 1 Nephi 19:10 (“the God of Abraham and of Isaac and the God of Jacob”) and in Helaman 3:30 (“with Abraham and Isaac and with Jacob and with all our holy fathers”); these two examples could well involve accidental omissions of repetitive elements, but we cannot be sure.