The printer’s manuscript doesn’t have the indefinite article a before remission, but the 1830 edition does. Since for this part of the text both these sources are firsthand copies of the original manuscript, we need to determine which variant was the more probable reading in 𝓞 (which is not extant here). If 𝓞 read a remission, then Oliver Cowdery must have accidentally omitted the a when he copied from 𝓞 into 𝓟. On the other hand, if 𝓞 read remission, then the 1830 compositor must have added the a when he set the type.
Elsewhere in the current text, remission usually occurs with a determiner—with either the indefinite article a (18 times) or the definite article the (4 times). Based on the earliest textual sources, there are also five occurrences without any determiner, of which three are of the specific form “repentance and remission of sins” (in 3 Nephi 7:16, 3 Nephi 7:23, and Moroni 3:3). At least in that expression, the lack of a determiner seems firm. The two other occurrences without a determiner could well have had an a in the original text; the original manuscript is not extant in either instance:
One important point to note here is that in none of these other instances of remission has there been any loss or gain of the indefinite article a: that is, no a has been lost from any of the other cases of a remission, even temporarily, in the manuscripts (or in the editions); nor for any of the other cases has an a ever been added in the editions (or in the manuscripts, even momentarily). So as far as the indefinite article for remission is concerned, we cannot rely on transmission errors to evaluate the case here in Helaman 14:13. But when we look at the addition and omission of the indefinite article a before other nouns, we find evidence that Oliver Cowdery sometimes dropped the a, even when the resulting language was clearly unacceptable:
In all three cases, Oliver’s manuscript corrections are not virtually immediate; in each case, there is a difference in the level of ink flow, which suggests a later correction. In fact, in one case (Alma 42:2), Oliver not only omitted the a in both 𝓞 and 𝓟 but he corrected only the error in 𝓞. In that case, the 1830 compositor supplied the necessary a.
On the other hand, there is one case (but only one) where the 1830 compositor added the indefinite article a (although unnecessarily), but in that case he was motivated to supply the a since the preceding text read a death:
In other words, the 1830 compositor rarely supplied an indefinite article a and then only when he would have been motivated to do so. Here in Helaman 14:13, the only motivation we can find for adding an a is that a remission is more frequent than remission alone. But there is clearly nothing wrong with the reading in 𝓟, “that thereby ye may have remission of them through his merits”. The 1830 compositor did not add any a to the two other instances of unconjoined remission (in Mosiah 3:13 and Moroni 8:25). To be sure, Oliver Cowdery never omitted an a from the 18 other cases of a remission. But since we do find evidence that Oliver occasionally omitted fully necessary instances of the indefinite article a, the critical text will accept the 1830 reading in Helaman 14:13 as the correct reading and assume that the reading in 𝓟 involved a loss of a. Basically, the odds are somewhat greater that Oliver Cowdery omitted the a than the 1830 compositor added it.
Summary: Retain the 1830 reading with a before remission in Helaman 14:13; scribal errors on Oliver Cowdery’s part suggest that he accidentally omitted the a in this case when he copied from 𝓞 into 𝓟; there is less evidence that the 1830 compositor would have added the a before remission when he set the type.