As mentioned under Mosiah 4:7, this is the only example in the text of even since; elsewhere there are examples of only ever since (seven of them). Moreover, there are quite a few examples in the manuscripts where Oliver Cowdery momentarily mixed up even and ever; for a list, see under Alma 56:46. In fact, one of these involves a case of ever since:
Thus it is quite possible that even in Helaman 8:19 is an early error for ever.
The text here in Helaman 8:19 emphasizes that prophets after Abraham testified of Christ’s coming. In fact, the immediately following text refers to some of these prophets by name:
And the immediately preceding text (in verses 16–18) refers not only to prophets that came between Abraham and Moses (listed under 1 below) but also to prophets that came before Abraham himself (listed under 2 below), yet all of them testifying of Christ:
Thus it seems wholly unnecessary for verse 19 to use even in referring to the prophets that came after Abraham (“even since the days of Abraham there hath been many prophets that hath testified these things”) given that these prophets (or at least the ones between Abraham and Moses) have already been mentioned in verse 16: “but also all the holy prophets from his day even to the days of Abraham” (for discussion of the lack of the word back in this phrase, see under Mosiah 28:17).
The word even might have worked here in verse 19 if the text had referred to “the days of Moses” rather than to “the days of Abraham” since those later prophets have not yet been specifically referred to here in the larger passage:
Of course, the prophets Isaiah and Jeremiah came after Moses. The prophets Zenos, Zenoch, and Ezaias are not identified in the Bible, so we cannot positively claim that they came after Moses, although the prophecies of Zenos and Zenoch are addressed to the house of Israel and refer to them being scattered and then eventually restored (1 Nephi 19:10–17, Jacob 5–6, and Alma 33:12–17). The specific prophecies of Zenos and Zenoch suggest that these two prophets were contemporary with prophets in Israel that lived prior to the destruction of Jerusalem by the Babylonians but after the Israelites had conquered the land of Canaan. (For discussion of when Ezaias might have lived, see nearby under Helaman 8:19–20.)
Although “the days of Abraham” in Helaman 8:19 could be an error for “the days of Moses”, it seems much more likely that even is a mistake for ever. Besides the many cases where these two words were mixed up in the manuscripts, there are several occurrences of even, all correct, in verses 16 and 18 that could have prompted Oliver Cowdery to accidentally write even in verse 19 instead of the correct ever:
In fact, in the case of Mosiah 13:33, cited above, the incorrect even since was probably prompted by the preceding (but correct) occurrence of even in the same sentence: “yea and even all the prophets which have prophesied even since the world began” (the initial reading in 𝓟).
The critical text will assume that here in Helaman 8:19 the earliest extant text, the printer’s manuscript, incorrectly reads “even since the days of Abraham”; the word even is very likely an error for ever, one that probably occurred as Oliver Cowdery took down Joseph Smith’s dictation or as he copied the text from 𝓞 into 𝓟.
Summary: Emend Helaman 8:19 to read “ever since the days of Abraham” since the emphasis here is on the prophets’ continual testifying of the coming of Christ, including prophets after Abraham; this emendation is supported by considerable manuscript evidence that Oliver Cowdery occasionally mixed up even and ever.