Oliver Cowdery initially wrote in the printer’s manuscript “to the baptizing”, but then he virtually immediately corrected the preposition to to unto by inserting un inline (but raised a little off the line); there is no change in the level of ink flow for the un.
Elsewhere in the text, gerundives are usually preceded by unto rather than to, but to is also possible. There are 44 other gerundives with unto in the original text, but there are 25 with to. For one other case of unto, Oliver initially wrote to in 𝓟; in this instance, 𝓞 is extant and it reads unto:
We can see the contrast in usage between unto and to for the gerundive phrase “(un)to the fulfilling of something”: there are 14 instances with unto and 4 with to. In general, either preposition is possible, so in each case we follow the earliest textual sources. Here in Helaman 3:26, we accept Oliver Cowdery’s correction in 𝓟 as the reading in 𝓞. For a general list of cases where Oliver wrote to instead of the correct unto, see under Jacob 2:17.
Summary: Maintain unto for the gerundive in Helaman 3:26, the corrected reading in 𝓟 (“unto the baptizing and uniting to the church of God many souls”).