In this verse, we have the only two occurrences of eyewitness in the Book of Mormon text. Here it appears that eyewitness is being used as an adjective rather than as a noun. If it were a noun, we would expect in modern English a preceding indefinite article an. Since both the 1830 edition and the printer’s manuscript are lacking the an in these two cases, we can be confident that the an was not in 𝓞 either. Of course, an original an could have been lost when the text was dictated to Oliver Cowdery. But the fact that it would have been twice lost in 𝓞 seems unlikely. Moreover, the use of eyewitness without an is unexpected, so the odds are that the original text read simply as eyewitness, the more difficult reading.
Another possibility, suggested by David Calabro (personal communication), is that eyewitness is a mishearing for a witness. In this passage, Oliver Cowdery could have twice misheard or misinterpreted Joseph Smith’s a /ß/ as eye /ßi/. In earlier English, the /ai/ diphthong was typically pronounced as /ßi/. In fact, this centralized pronunciation is still found in dialects of American English, especially among rural and less educated speakers in New England. Elsewhere in the text, there are seven instances of a witness, but none of these have shown any tendency to be replaced by eyewitness. More generally, there are no scribal mix-ups in the manuscripts involving the sounds /ai/ and /ß/, so this potential error seems unlikely. In addition, eyewitness is the unexpected reading (in comparison with a witness), so it seems unlikely that an original a witness would have been twice replaced by eyewitness during the dictation of the text (or that the expected an eyewitness would have been twice replaced by the unexpected eyewitness, as discussed above).
For discussion of the pronunciation /ßi/ in Early Modern English, see pages 104–109 of Charles Barber’s Early Modern English (Edinburgh: Edinburgh University Press, 1997). For this and similar pronunciations in New England dialects, see pages 526–527 in volume 3 of J. C. Wells’ Accents of English (Cambridge: Cambridge University Press, 1982). I wish to thank Don Chapman for help in locating these references.
Summary: Accept in 3 Nephi 7:15 the two instances of eyewitness; in this passage eyewitness acts as an adjective rather than as a singular noun, and thus the expected indefinite article an is lacking; eyewitness is probably not an error for a witness since there is no evidence for mix-ups between the sounds /ai/ and /ß/ elsewhere in the history of the Book of Mormon text.