In copying from 𝓞 into 𝓟, Oliver Cowdery seems to have initially omitted the thereof that came after the fall. Oliver caught his error and supralinearly inserted the thereof with slightly heavier ink flow. It looks like he redipped his quill just before making the correction. The usage here parallels the language in 1 Nephi 11:36 (“the great and spacious building was the pride of the world and the fall thereof was exceeding great”).
Another possibility, however, is that the original text here in 2 Nephi 18:18 read “and great must be the fall of it”, as in another passage:
In fact, both 1 Nephi 22:14 and 2 Nephi 28:18 use virtually the same language to refer to the fall of that great and abominable church. Thus it is possible that the original manuscript (which is not extant here in 2 Nephi 28:18) defectively read “and great must be the fall” (that is, in 𝓞 there was no postmodifier after the fall ), with the result that Oliver Cowdery emended the printer’s manuscript by inserting thereof, when another possibility would have been to insert of it, as in 1 Nephi 22:14.
The fact that the ink flow in 𝓟 for the supralinearly inserted thereof is only slightly heavier argues that this correction was simply the result of trying to accurately copy the text from 𝓞 into 𝓟. Elsewhere, Oliver Cowdery only once had a problem in writing down thereof; in that one instance, it seems that he initially started to write of it instead of the correct thereof:
Oliver’s correction here is immediate (the of is erased and the correct thereof is written immediately afterwards inline). If anything, this example of a scribal slip shows that Oliver’s natural tendency was to supply of it rather than thereof. Therefore, the inserted thereof in 2 Nephi 28:18 is perhaps not what Oliver would have naturally supplied on his own.
Summary: Maintain in 2 Nephi 28:18 the corrected reading in 𝓟: “and great must be the fall thereof ”.