Here we have two examples in 𝓞 where Oliver Cowdery initially omitted the and as he took down Joseph Smith’s dictation. In the first case, Oliver inserted an & with heavier and broader ink flow. The insertion is slightly elevated from the original line and is therefore inserted between the words footstool and he. The second example of an inserted & is also slightly elevated between the words (here Abraham and Isaac), although in this second case the correction was done with a very weak ink flow, with the result that the & was not noticed when Oliver copied 𝓞 into 𝓟. (Close examination of the original manuscript, especially using multispectral imaging as well as ultraviolet photographs, shows that there is an ampersand inserted after Abraham in 𝓞.) On the other hand, the first example of the inserted & was copied into 𝓟 since that ampersand was written with heavier ink flow.
Neither of these insertions is an immediate correction. In the first case, it appears that Oliver Cowdery dipped his quill before writing in the & in the second case, it appears that he had little ink in his quill. The clear difference in ink flow implies that the two ampersands were inserted at different times.
The heavier ink correction for the first & is also found for another non-immediate correction, namely Oliver Cowdery’s correction of his to this near the end of verse 39. Here Oliver initially wrote “for it is his throne and his earth is his footstool”. Obviously, Oliver mistakenly wrote “his earth” due to the influence of the nearby “his throne” and “his footstool”. The correction of the his to this (the t is inserted right in front of the h of the his) looks like it was made at about the same time the & was inserted after footstool.
In both these cases involving the inserted &, the motivation for editing the text is not strong. Note that the sentence that begins verse 39 has no initial and (“he ruleth high in the heavens”), so the sentence that begins verse 40 could have also had no initial and. And in the case of “Abraham (and) Isaac and Jacob”, English speakers prefer no and between the first two names. This preference was probably responsible for Oliver Cowdery accidentally dropping the and between Abraham and Isaac when he initially wrote down 1 Nephi 17:40. Elsewhere, the text has examples with and without this extra and:
Thus variation is possible. Ultimately, there seems to be no motivation for Oliver to have inserted the ampersand between Abraham and Isaac except that this and was part of the original text that Joseph Smith dictated to him.
Summary: Maintain in 1 Nephi 17:39–40 the pair of corrections in 𝓞 involving heavier ink flow, namely this for his before earth and the and that begins verse 40 (“and he loveth them”); based on the weakly inserted & in 𝓞, the and between Abraham and Isaac in 1 Nephi 17:40 should be restored.