Here Oliver Cowdery initially wrote the singular tradition, then later added the plural s. The ink flow for the s is considerably heavier and darker, just like the s that he inserted at the end of the word thousand on the previous page of 𝓟 (see the discussion under the Words of Mormon 1:14). Throughout this part of 𝓟, Oliver used the same heavier flowing and darker ink to correct 𝓟. See, for instance, the discussion regarding the two supralinear insertions in Omni 1:6, three manuscript pages earlier (on page 113 of 𝓟). The first of those two corrections clearly indicates a correction to 𝓞. Thus the scribal evidence in this part of 𝓟 suggests that here in Mosiah 1:5 Oliver was proofing 𝓟 against 𝓞 when he added the s to tradition. The critical text will therefore accept the corrected reading in 𝓟 since it appears to be due to proofing, although the possibility of editing cannot be ruled out.
The use of the plural are in the following relative clause could be used to argue that the plural is correct (“the traditions of their fathers which are not correct”). Even so, the immediately preceding plural fathers (in the prepositional phrase “of the fathers”) could have led to the choice of the plural are. For an example of such a proximity effect, see Jarom 1:14, where the earliest text read “the record of our wars are engraven”. Another example is found in the Words of Mormon 1:4, which originally read “as many things as have been prophesied concerning us down to this day has been fulfilled”. See subject-verb agreement in volume 3 for additional discussion of cases where the relative clause has a plural verb form that agrees with a closer plural noun rather than with the relative pronoun’s singular antecedent.
In the Book of Mormon text, there are two other cases of the phrase “tradition(s) of one’s fathers” that involve variation; both of these can be interpreted as cases where the immediate proximity of the preceding fathers may have influenced the verb form in a following relative clause:
In the first example, the incongruity between the singular tradition and the plural were has been corrected in the LDS text, beginning with the 1852 edition. In the second example, the original manuscript initially had the plural traditions, but Oliver Cowdery immediately erased the plural s, thus producing the singular tradition. This singular agrees, in the original manuscript, with the third person singular form leads in the following relative clause. Interestingly, in the printer’s manuscript, Oliver replaced leads with lead, probably because of the preceding fathers (“the tradition of your fathers which lead you away”). Another possibility for this second case, pointed out by David Calabro (personal communication), is that Oliver’s lead in 𝓟 may have actually stood for the past-tense form led, especially since Oliver tended to spell led as lead in both manuscripts (33 out of 40 times in extant portions of 𝓞; 59 out of 92 times in 𝓟). See Alma 21:17 and Alma 30:16 for discussion of these two Alma passages involving variation; also see the discussion under subject-verb agreement in volume 3.
There are also four other examples in the manuscripts where Oliver Cowdery varied the number for the word tradition. All of these cases are extant in 𝓞 and involve manuscript corrections. In one case, the correction is in 𝓞 itself; in the three other cases, 𝓞 confirms that the correction in 𝓟 was based on the reading in 𝓞:
Elsewhere in the text—and without variation in the manuscripts and the printed editions—there are a good many cases of both “the tradition of one’s fathers” (9 occurrences) and “the traditions of one’s fathers” (16 occurrences). So either reading is possible. In the seven cases involving textual variation, the safest solution is to rely on the evidence from the earliest textual sources, even if in one case it leads to subject-verb disagreement (namely, in Alma 21:17: “the tradition of their fathers which were not correct”).
Summary: Accept in Mosiah 1:5 the corrected reading in 𝓟 (“the traditions of their fathers which are not correct”); although the plural s for traditions is in heavier and darker ink, it looks the same as other corrections in this part of 𝓟 that appear to be based on 𝓞 and not due to editing.