Oliver Cowdery was not able to figure out the word scribe 2 wrote down in 𝓞 to describe the devil’s connection with hell. Oliver interpreted the word as preparator. The 1830 compositor set this word. In his editing for the 1837 edition, Joseph Smith did not like the word preparator. He first thought of father as a possible replacement and wrote it supralinearly after crossing out preparator. Joseph then changed his mind and replaced father with foundation. In selecting this word, he was probably influenced by the earlier passages in 1 Nephi 13–14 which he had recently edited so that Satan would be the foundation (rather than the founder) of the great and abominable church (see the discussion under 1 Nephi 13:6). The 1981 LDS edition restored the earlier preparator.
In the mid-1990s, I had Renee Bangerter, one of my research assistants, make a search in Martin Lehnert’s Reverse Dictionary of Present-Day English (Leipzig: VEB Verlag Enzykolopädie, 1971) for all nouns ending in -tor to see what other possible words might look like preparator. She found the following phonetically close candidates: perpetuator, perpetrator, appropriator, procreator, and proprietor, with the last being the most plausible.
Upon reexamination of the handwriting for this word in the original manuscript, I discovered that scribe 2 of 𝓞 had indeed written proprietor but had spelled it as prepriator. Unfortunately, scribe 2 of 𝓞 had made it very difficult for Oliver Cowdery to figure out the word. First of all, scribe 2’s pr looks more like pe, a characteristic of his handwriting. See for instance, line 24 on this page of 𝓞 (page 29), where scribe 2 wrote the pr in probation this same way. Besides that difficulty, scribe 2 also overwrote the middle part of the word with an a in heavy ink flow, thus further obscuring the intended word. Given this confusion, Oliver was unable to recover the word as proprietor. So he had to guess the word; he could tell that it began with a p and ended in -ator. Bangerter further proposed that Oliver’s guess, preparator, was influenced by the word prepared found in the previous line of 𝓞 (“there is a place prepared”), and thus Oliver interpreted the word as preparator, a virtually nonexistent word in English. In the Oxford English Dictionary, preparator is listed as meaning ‘a preparer of medicines or specimens’. (For Bangerter’s discussion, see pages 56–57 of her 1998 master’s thesis, Since Joseph Smith’s Time: Lexical Semantic Shifts in the Book of Mormon.)
Another possibility, perhaps more plausible semantically, would have been to interpret the unknown word as preparer. Nonetheless, the preceding references to hell as being prepared for the wicked never states that the devil himself prepared the place. In fact, we should rather think that the Lord prepared the place. But the devil could well be said to be the proprietor of hell.
The word proprietor does not occur elsewhere in the Book of Mormon text proper, although the word was used in the copyright statement for the Book of Mormon when Joseph Smith (in accord with copyright language) identified himself as the book’s “author and proprietor”. Joseph Smith would have known the word proprietor, but it seems unlikely that he would have known the specific word preparator, although he could have guessed that it meant ‘a preparer’.
Summary: Emend 1 Nephi 15:35 to read that “the devil is the proprietor” of hell; the word in scribe 2’s hand in the original manuscript apparently reads prepriator (that is, proprietor), which Oliver Cowdery misread as preparator under the influence of the preceding prepared.